Mechanism of returning to neutral for axle driving apparatus

ABSTRACT

A hydrostatic transmission including a hydraulic pump and a hydraulic motor fluidly connected with each other, being immersed in an oil sump formed in a housing. A pair of oil passages for fluidly connecting the hydraulic pump with the hydraulic motor are formed in a center section on which the hydraulic pump and the hydraulic motor are mounted. The center section is provided on an outer surface thereof with a pair of openings of the respective oil passages. A control arm is disposed in the housing so as to interlock with a pump capacity changing member like a movable swash plate, thereby enabling the pump capacity changing member to be moved to various actuation positions including a neutral position in association with rotation of the control arm rotated to various actuation positions including a neutral position. A contact member having a surface abutting against the pair of openings of the center section is formed integrally with the control arm. A pair of oil passages are provided in the contact member. The pair of oil passages in the contact member are shut from the oil sump and open on the surface of the contact member. The pair of oil passages in the contact member communicate with the respective openings of the center section when the control arm is located in a certain range including the neutral position. An orifice is provided in the contact member so as to connect the pair of oil passages in the contact member with each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of application Ser. No. 09/691,137, filed Oct. 19, 2000, now U.S. Pat. No. 6,349,544, which is a Continuation-in-Part of application Ser. No. 09/551,532; filed Apr. 18, 2000, now U.S. Pat. No. 6,341,488 which is a Continuation-in-Part of application Ser. No. 09/192,315; filed Nov. 16, 1998, now U.S. Pat. No. 6,109,032 which is a Continuation-in-Part of application Ser. No. 08/872,672; filed Jun. 11, 1997, now U.S. Pat. No. 5,836,159. The disclosures of each of the above-identified applications are incorporated in their entirety by reference hereto.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mechanism for automatically returning a movable swash plate to the neutral position in an axle driving apparatus which drives the axles thereof by a hydrostatic transmission (hereinafter referred to as an “HST”) for steplessly changing the rotational speed of an engine, wherein resistance is given to the movable swash plate and pressure generated in a closed fluid circuit of the HST escapes therefrom when the movable swash plate returns to the neutral position, so as to prevent the vehicle from a sudden stop.

2. Background Art

Conventionally, an HST comprising a hydraulic pump and a hydraulic motor, which are fluidly connected through a closed fluid circuit, has a control arm which engages with a movable swash plate of the hydraulic pump. The quantity of oil discharged from the hydraulic pump can be changed in a stepless fashion by rotation of the control arm. The control arm is regularly biased by a neutral return spring, whereby the movable swash plate is automatically returned to its neutral position by the neutral return spring when an operator stops speed changing operation and releases the operational power. Accordingly, the movable swash plate suddenly returns to the neutral position when the operating force is released at a high speed position of the movable swash plate and operating oil which has smoothly circulated in the closed fluid circuit theretofore is blocked in the hydraulic pump so that the rotation of the hydraulic motor is suddenly stopped. In other words, a dynamic brake is applied. Therefore, a control arm is often provided with a shock absorber to prevent sudden returning to its neutral position, as disclosed in U.S. Pat. No. 5,094,077, for example.

A shock absorber which is provided to prevent the control arm from suddenly returning to its neutral position is disposed on the outside of the HST housing so as to enable mounting and exchange thereof. However, a shock absorber which uses gas or fluid for shock absorption is heated by radiation of the housing which is heated by the rising temperature of the oil therein and is affected by the temperature of the outside air. The gas or fluid expands or contracts because of such a change of temperature so that operation of the shock absorber differs according to temperature or, in some cases, the neutral position can not be fixed at a high or a low temperature.

Further, a shock absorber mounted on the outside of the housing requires space for it to be mounted which can restrict the shape of the axle driving apparatus. As a result, the entire axle driving apparatus becomes larger. Also, because the shock absorber is mounted outside of the housing, the movable portion of the shock absorber is conventionally covered with a protector such as a rubber boot to avoid penetration of water, dust or other foreign matter, and therefore, requires an increase in the number of parts and in the cost.

BRIEF SUMMARY OF THE INVENTION

The main object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a shock of deceleration by returning to neutral in advancing or reversing of a vehicle.

The second object of the present invention is to provide a mechanism of returning to neutral for an axle driving apparatus, which can moderate a starting shock of vehicle for advancing and reversing.

The third object of the present invention is to provided a mechanism of returning to neutral for an axle driving apparatus, which can prevent the vehicle left on a slope in neutral from suddenly self-descending.

The axle driving apparatus regarding to the present invention is constructed as follows:

Axles and a hydrostatic transmission as a combination of a variable capacity hydraulic pump and a hydraulic motor for driving axles are contained in fluid sump formed within a housing for the axle driving apparatus. The hydrostatic transmission is constructed such that suction and discharge ports of the hydraulic pump are respectively connected with discharge and suction ports of the hydraulic motor through fluid passages. The hydrostatic transmission is provided with a capacity changing member, which can be shifted between a neutral position making the hydraulic pump discharge substantially no fluid and an acting range making the hydraulic pump discharge fluid. A biasing member biases the capacity changing member in the acting range so as to return it to the neutral position. Orifices are provided which are connected to the fluid passages. A shutting member faces the orifices so as to be operable to open and shut the orifices toward the fluid sump. The shutting member is interlocked with the capacity changing member so that at least one of the orifices is open to the fluid sump during returning of the capacity changing member from the acting range to the neutral position. Pistons with the orifices provided therethrough are slidably fit to the oil passages and the outer surfaces thereof are pressed against the shutting member by hydraulic pressure in the oil passages. The shutting member is provided with grooves so as to communicate with the orifices of the piston pressed as such. Other biasing members for pushing the pistons are smaller than the biasing member for returning the capacity changing member to neutral.

In such a construction, to achieve the main object, a predetermined position is provided in the acting range, so that when returning the capacity changing member in the acting range to the neutral position, the shutting member shuts both the orifices of the pistons until it reaches the predetermined position, and after the capacity changing member passes the predetermined position until the neutral position, it allows one of the orifices to open toward the fluid sump while still shutting the other orifice.

Alternatively, the shutting member is constructed such that during returning of the capacity changing member in the acting range to the neutral position, it shuts one of the orifices and allows the other orifice to open toward the fluid sump at any position in the acting range.

To achieve the second object, the shutting member is constructed such that at the moment that the capacity changing member leaves the neutral position, it allows both the orifices to open toward the fluid sump, and when the capacity changing member is further apart from the neutral position, the shutting member shuts one of the orifices.

To achieve the third object, the shutting member is constructed such that when the capacity changing member is in the neutral position, it allows both the orifices to open partly toward the fluid sump, or it allows both the orifices to open fully in case that the orifices are diametrically small enough, or the shutting member shuts both the orifices in case that another member for giving a dead zone of the neutral position is provided, or it shuts one of the orifices.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a plan view, partly in cross section, of an axle driving apparatus of the present invention, from which an upper half housing has been removed;

FIG. 2 is a cross-sectional view looking in the direction of arrows 2—2 in FIG. 1;

FIG. 3 is a cross-sectional view looking in the direction of arrows 3—3 in FIG. 1;

FIG. 4 is a cross-sectional view looking in the direction of arrows 4—4 in FIG. 1;

FIG. 5 is a plan view of center section 5 of the present invention;

FIG. 6 is a view looking in the direction of arrow 6 in FIG. 5;

FIG. 7 is a cross-sectional view looking in the direction of arrows 7—7 in FIG. 6;

FIG. 8 is a cross-sectional view looking in the direction of arrows 8—8 in FIG. 5;

FIG. 9 is a cross-sectional view looking in the direction of arrows 9—9 in FIG. 5;

FIG. 10 is a partial cross-sectional side view of the axle driving apparatus of the present invention showing a side view of operational members of movable swash plate 11;

FIG. 11 is a perspective view of control arm 39;

FIG. 12 is a cross-sectional view looking in the direction of arrows 12—12 in FIG. 10;

FIG. 13 is a cross-sectional view of a principal part of another embodiment of the present invention showing the disposition of pistons 64 in center section 5;

FIG. 14 is a cross-sectional view of a principal part of a further embodiment of the present invention showing the shape of groove 39 d on the surface of control arm 39 in contact with piston 64;

FIG. 15 is a plan view of center section 5′;

FIG. 16 is a cross-sectional plan view of center section 5′;

FIG. 17 is a cross-sectional view looking in the direction of arrows 17—17 in FIG. 15;

FIG. 18 is a cross-sectional view looking in the direction of arrows 18—18 in FIG. 15;

FIG. 19 is a cross-sectional view looking in the direction of arrows 19—19 in FIG. 15;

FIG. 20 is a side view looking in the direction of arrow 20 in FIG. 15;

FIG. 21 is a cross-sectional plan view of center section 5″;

FIG. 22 is a cross-sectional view looking in the direction of arrows 22—22 in FIG. 21;

FIG. 23 is a vertical rear elevation view of another embodiment of an axle driving apparatus containing center section 5″ of FIG. 21;

FIG. 24 is a cross-sectional view looking in the direction of the arrows 24—24 in FIG. 23; and

FIG. 25 is a perspective view of a movable swash plate 11′ for the axle driving apparatus shown in FIG. 23.

FIG. 26 is a plan view, partly in cross section, of another axle driving apparatus of the present invention suiting center section 5′ shown in FIGS. 15 through 20 in condition that upper half housing 1 has been removed therefrom;

FIG. 27 is a cross-sectional view looking in the direction of arrows 27—27 in FIG. 26, being provided with upper half housing 1;

FIG. 28 is a cross-sectional view looking in the direction of arrows 28—28 in FIG. 26, being provided with upper half housing 1;

FIG. 29 is a cross-sectional view of a principal part of the axle driving aparatus shown in FIGS. 26 through 28 showing the disposition of upper and lower pistons 64U and 64L in center section 5′;

FIG. 30 is a perspective view of a preferred first embodiment of a control arm 39′ which is suitable to center section 5′ with pistons 64U and 64L;

FIG. 31 is a side view of the same in a neutral position N;

FIGS. 32(a) through 32(d) are views showing the same control arm 39′ at various positions in series of its advancing range A;

FIGS. 33(a) (e) (f)-(g) are views showing the same control arm 39′ at various positions in series of its reversing range R;

FIG. 34 is a hydraulic circuit diagram of an HST provided with center section 5′ and control arm 39′ according to the first embodiment shown in FIGS. 31, 32, 36, 37, 39 and 40;

FIG. 35 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the preferred first embodiment of control arm 39′ shown in FIGS. 31 and 32;

FIG. 36 is a side view of a first modification of the first embodiment of control arm 39′ in neutral position N;

FIG. 37 is a side view of a second modification of the first embodiment of control arm 39′ in neutral position N;

FIG. 38 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the second modification of the first embodiment control arm 39′ shown in FIG. 37;

FIG. 39 is a side view of a third modification of the first embodiment of control arm 39′ in neutral position N;

FIG. 40 is a side view of a fourth modification of the first embodiment of control arm 39′ in neutral position N;

FIG. 41 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the third and fourth modifications of the first embodiment of control arm 39′ shown in FIGS. 39 and 40;

FIG. 42 is a perspective view of a preferred second embodiment of control arm 39′ which is suitable to center section 5′ with pistons 64U and 64L;

FIG. 43 is a hydraulic circuit diagram of an HST provided with center section 5′ and control arm 39′ according to the second embodiment shown in FIGS. 42, 45 and 47;

FIG. 44 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the preferred second embodiment of control arm 39′ shown in FIG. 42;

FIG. 45 is a side view of a first modification of the second embodiment of control arm 39′ in neutral position N;

FIG. 46 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the first modification of the second embodiment of control arm 39′ shown in FIG. 45;

FIG. 47 is aside view of a second modification of the second embodiment of control arm 39′ in neutral position N;

FIG. 48 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the second modification of he second embodiment of control arm 39′ shown in FIG. 47;

FIG. 49 is a side view of a preferred third embodiment of control arm 39′ which is suitable to center section 5′ with pistons 64U and 64L;

FIG. 50 is a hydraulic circuit diagram of an HST provided with center section 5′ and control arm 391 according to the third embodiment shown in FIGS. 49, 52 and 54;

FIG. 51 is a schematic diagram showing opening conditions of orifices 60U and 80L in relation to the rotational positions of the preferred third embodiment of control arm 39′ shown in FIG. 49;

FIG. 52 is a side view of a first modification of the third embodiment of control arm 39′ in neutral position N;

FIG. 53 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the first modification of the third embodiment of control arm 39′ shown in FIG. 52;

FIG. 54 is a side view of a second modification of the third embodiment of control arm 39′ in neutral position N;

FIG. 55 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the second modification of the third embodiment of control arm 39′ shown in FIG. 54;

FIG. 56 is a side view of a preferred fourth embodiment of control arm 39′ which is suitable to center section 5′ with pistons 64U and 64L;

FIG. 57 is a hydraulic circuit diagram of an HST provided with center section 5′ and control arm 39′ according to the fourth embodiment shown in FIG. 56;

FIG. 58 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the preferred fourth embodiment of control arm 39′ shown in FIG. 56;

FIG. 59 is a side view of a fifth embodiment of control arm 39′;

FIG. 60 is a perspective view of control arm 39′ of FIG. 59;

FIG. 61 is a sectional front view of a principal portion of the axle driving apparatus which employs the fifth embodiment of control arm 39′;

FIG. 62 is a schematic diagram showing opening conditions of orifices 80U and 80L in relation to the rotational positions of the fifth embodiment of the control arm 39′;

FIG. 63 is a cross-sectional view of a principal part of the axle driving apparatus shown in FIGS. 26 through 28, showing that upper and lower pistons 64U and 64L are provided in series thereto with respective upper and lower plate-type check valves 92U and 92L for stopping air bubbles generated from the oil sump into center section 5′;

FIG. 64 is a cross-sectional view of the same, showing that upper and lower pistons 64U and 64L are provided in series thereto with respective upper and lower ball-type check valves 95U and 95L for stopping air bubbles generated from the oil sump into center section 5′;

FIG. 65 is a cross-sectional view of the same, showing that upper and lower pistons 64U and 64L are provided therein with respective upper and lower ball-type check valves 95U and 95L for stopping air bubbles generated from the oil sump into center section 5′;

FIG. 66 is a side view of a plate-type check valve 92 when viewed from a second opening 93 b thereof to a first opening 93 a thereof; and

FIG. 67 is a side view of a ball-type check valve 95 when viewed from a second opening 96 b thereof to a first opening 96 a thereof.

FIG. 68 is a sectional view of a principal portion of the axle driving apparatus shown in FIGS. 26 to 28, showing a structure according to a sixth embodiment of the present invention, wherein a pair of oil through-passages 98U and 98L are formed in a contact plate 39′c of control arm 39′ for leading the hydraulic oil in oil passages 5′a and 5′b in the center section to a back side of contact plate 39′c and connected to each other through an orifice 100 a, and a plate spring 104 is disposed on a portion of friction plate 68′ abutting against the back side of contact plate 39′c for prevention of air bubbles from entering oil passages 5′a and 5′b and for leading the hydraulic oil from oil passages 5′a and 5′b into friction plate 68′ when its hydraulic pressure is excessively increased;

FIG. 69 is a sectional front view of control arm 39′, wherein both oil through-passages 98U and 98L are communicated with each other through the orifice;

FIG. 70 is an exploded perspective view of contact plate 39′c of control arm 39′ and an orifice member 100 during assembly thereof, wherein oil through-passages 98U and 98L are formed in contact plate 39′c;

FIG. 71 is an exploded perspective view of friction plate 68′ and plate spring 104 during assembly thereof;

FIG. 72 illustrates front views of control arm 39′ being operated for advancing the vehicle, showing conditions of communication between oil supply/discharge ports 82U and 82L and respective oil through-passages 98U and 981L;

FIG. 73 illustrates front views of control arm 39′ being operated for reversing the vehicle, showing conditions of communication between oil supply/discharge ports 82U and 82L and respective oil through-passages 98U and 98L;

FIG. 74 is a hydraulic oil circuit diagram of the EST constructed as shown in FIG. 68;

FIG. 75 is a sectional view of a hydraulic oil circuit structure as a first modification of the sixth embodiment shown in FIG. 68, wherein a valve is provided in the orifice communicating oil through-passages 98U and 98I,

FIG. 76 is an enlarged view of a valve portion in the structure shown in FIG. 75;

FIG. 77 is a hydraulic oil circuit diagram of the EST according to the first modification of the sixth embodiment;

FIG. 78 is a sectional view of a hydraulic oil circuit structure as a second modification of the sixth embodiment shown in FIG. 68, wherein plate spring 104 is located on the back side of control arm 39′ and a pair of check valves 113U and 113L are disposed on plate spring 104;

FIG. 79 is a sectional front view of control arm 39′, both oil through-passages 98U and 98L are communicated with each other through an orifice 114;

FIG. 80 is an exploded perspective view of contact plate 39′c of control arm 39′, a round rod 100′ for forming orifice 114, plate spring 104 and check valves 113U and 113L during assembly thereof, wherein oil through-passages 98U and 98L are formed in contact plate 39′c; and

FIG. 81 is a hydraulic oil circuit diagram of the HST according to the second modification of the sixth embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Explanation will first be given on the entire construction of an axle driving apparatus having the neutral return mechanism of the present invention. FIGS. 1, 2, 3 and 4 show a housing constructed by joining an upper half housing 1 and a lower half housing 2 along horizontal joint surfaces. At the joint surfaces is provided a bearing for a motor shaft 4. Bearings for axles 7 are shifted upwardly from the joint surfaces of the housing and are disposed in upper half housing 1 to rotatably support axles 7. Axles 7 are differentially coupled by a differential gear unit 23. One end of each axle 7 projects laterally from the housing.

The interior of the housing is divided by an inner wall 8 into a first chamber R1 for housing an HST and a second chamber R2 for housing differential gear unit 23, a transmission gear unit for transmitting power from motor shaft 4 to differential gear unit 23 and axles 7. First and second chambers R1 and R2 are filled with lubricating oil in common so as to form an oil sump. An air reservoir (not shown) is formed above differential gear unit 23 in upper half housing 1. An oil supply hole is bored through the portion of the housing which is above the air reservoir, enabling oil to be supplied thereto.

Within first chamber R1 is mounted a center section 5 which is L-like shaped when viewed from the side and has a horizontal portion 500 and a vertical portion 501. At the peripheral portions of horizontal portion 500 are vertically open through bores 5f at three positions, as shown in FIG. 5. A mounting bolt 30 is inserted into each through bore 5 f from below to fix center section 5 to the inside of upper half housing 1. On the upper surface of horizontal portion 500 of center section 5 is formed a pump mounting surface 40. A cylinder block 16 is rotatably and slidably disposed thereon. Pistons 12 are fitted, through biasing springs, into a plurality of cylinder bores in cylinder block 16 and are reciprocally movable. A thrust bearing 11 a of a movable swash plate 11 abuts against the heads of pistons 12. An opening 11 b is provided at the center of movable swash plate 11 so as to enable a pump shaft 3 to perforate therethrough. Pump shaft 3 is also used as an input shaft and is vertically disposed and is not relatively rotatably retained onto the axis of rotation of cylinder block 16, thereby constituting an axial piston type hydraulic pump. Pump shaft 3 projects outwardly at the upper end thereof from upper half housing 1. An input pulley 43, with a cooling fan 44, is fixed onto pump shaft 3. Input pulley 43 receives power from a prime mover (not shown) through a belt transmitting mechanism (also not shown).

As shown in FIG. 6, at the outside surface of vertical portion 501 of center section 5 is formed a motor mounting surface 41 on which a cylinder block 17 is rotatably supported. A plurality of pistons 13 are fitted into a plurality of cylinder bores in cylinder block 17. Pistons 13 are reciprocally movable whereby the heads thereof abut against a fixed swash plate 37 which is fixedly sandwiched between upper half housing 1 and lower half housing 2. Motor shaft 4 is horizontally disposed on the axis of rotation of cylinder block 17 and is not relatively rotatably retained thereto so as to constitute an axial piston type hydraulic motor. Motor shaft 4 is also rotatably supported by a bearing bore provided on vertical portion 501 of center section 5 and by a bearing 4 a with a seal held at the joint surfaces of upper half housing 1 and lower half housing 2.

Transmission gear unit for transmitting power from motor shaft 4 to differential gear unit 23 is shown in FIGS. 1 and 2. A gear 25 engageable with a larger diameter gear 24 on counter shaft 26 is provided on motor shaft 4 where it enters into second chamber R2. A smaller diameter gear 21 on counter shaft 26 engages with a ring gear 22 of differential gear unit 23. Ring gear 22 drives differential gear unit 23 so as to transmit power to left and right axles 7.

As shown in FIG. 2, a brake disk 19 is fixed onto an axial end of motor shaft 4 positioned in second chamber R2. A brake operating shaft 14 is supported by upper half housing 1 through a bush 15. A brake arm 27 is fixed to the outer end of brake operating shaft 14 projecting from the housing. When brake arm 27 is rotated, balls 20 ride on cam grooves provided on a flange 14 a. Brake operating shaft 14 slides toward brake disk 19, so that the brake disk 19 is put between the inner end surface of brake operating shaft 14 and a brake pad 29, thereby exerting a braking action to motor shaft 4.

As shown in FIG. 5, a pair of arcuate ports 40 a and 40 b are open along pump mounting surface 40 on horizontal portion 500 of center section 5 so that the feed oil discharged from cylinder block 16 is introduced into center section 5. As shown in FIG. 6, a pair of arcuate ports 41 a and 41 b are open on the motor mounting surface 41 of vertical portion 501 thereby introducing feed discharge oil into center section 5 from cylinder block 17.

As shown in FIG. 7, a linear first oil passage 5 a and a linear second oil passage 5 b are drilled in parallel with each other within the thick horizontal portion 500 of center section 5 forming a closed fluid circuit for circulating operating oil between the hydraulic pump and the hydraulic motor. As shown in FIGS. 8 and 9, oiling pipes 52 and 53 communicating with first and second oil passages 5 a and 5 b are disposed on the lower surface of horizontal portion 500 of center section 5 and are exposed at the lower ends thereof at the outer bottom surface of lower half housing 2. After the axle driving apparatus has been assembled, the closed fluid circuit is filled with operating oil from the open end of each oiling pipe 52 and 53. Thereafter, the open end of each oiling pipe 52 and 53 is closed by a plug.

As shown in FIGS. 3, 7, 8 and 9, oil holes 5 c and 5 d are branched downwardly from the intermediate portion of first and second oil passages 5 a and 5 b. Oil holes 5 c and 5 d are open on the lower surface of horizontal portion 500 of center section 5. Check valves 54 and 55, which automatically open merely when oil is supplied, are disposed in the open end of oil holes 5 c and 5 d, respectively. As shown in FIG. 3, valve casings 54 a and 55 a are supported on projections 2 a which project upwardly from the inner bottom surface of lower half housing 2. An annular oil filter 56 is disposed in the oil sump between the lower surface of center section 5 and the inner bottom surface of lower half housing 2 and surrounds projections 2 a.

As shown in FIGS. 2 and 4, a by-pass operating arm 60 is disposed on upper half housing 1 so as to open first and second oil passages 5 a and 5 b into the oil sump for enabling axles 7 to be idle when the vehicle is hauled. In particular, by-pass operating arm 60 is fixed at the base thereof to an upper end of a by-pass shaft 61 which is vertically and pivotally supported to the upper wall of upper half housing 1. By-pass shaft 61 extends at the lower end thereof into vertical portion 501 of center section 5, so that a flat surface 61 a formed at the side surface of the lower end thereof abuts one end of a push pin 62 which can abut at the other end thereof against the rotatable slidable surface of cylinder block 17 supported on vertical portion 501. When an operator operates by-pass operating arm 60 outside of the housing for hauling the vehicle, by-pass shaft 61 is rotated and flat surface 61 a at the lower end thereof diagonally presses push pin 62 toward cylinder block 17. First and second oil passages 5 a and 5 b communicate with the oil sump in the housing through arcuate ports 41 a and 41 b, thereby enabling motor shaft 4 to be idle.

As shown in FIGS. 7, 8, 9 and 12, pistons 64 constituting the neutral return means of the present invention are horizontally and slidably disposed in the open end portions of first and second oil passages 5 a and 5 b which are open at the side surface of horizontal portion 500 of center section 5, opposite to vertical portion 501. Each piston 64 is cylindrical and has a large diametric flange 64 a formed at the outside end thereof. Each piston 64 is slidably inserted into an axial opening through a bush 65 which is screwed into the open end of each of first and second oil passages 5 a and 5 b. As best seen in FIG. 12, a seal 66 is disposed between each bush 65 and the external surface of piston 64. A seal 67 is disposed between each bush 65 and center section 5. Seals 66 and 67 prevent oil from leaking. An oil passage 64 b is bored along the axis of piston 64. A discharging oil hole 64 c having a diameter smaller than oil passage 64 b, is formed in the closed end of oil passage 64 b. Each oil passage 5 a and 5 b can be opened to the outside of the closed fluid circuit through oil passage 64 b and discharging oil hole 64 c.

Movable swash plate 11 is constructed for slanting movement. As shown in FIGS. 1, 2 and 10, a bush 51 is disposed on a side wall of upper half housing 1 positioned on a phantom extension line of the center of curvature X of convex portion 11 c at a back surface of movable swash plate 11. Bush 51 rotatably supports a control shaft 35. Onto the outer end of control shaft 35, outside of the housing is fixed a control lever 38 to enable movable swash plate 11 to be slantingly operated from the exterior of the housing. Control lever 38 is connected through a control rod (not shown) and may be pushed or pulled longitudinally to control a speed changing member (not shown) of the vehicle, such a lever or a pedal (not shown) provided on the vehicle.

As shown in FIG. 10, a control arm 39 is fixed to the inner end of control shaft 35 within the housing, and comprises a first arm 39 a, a second arm 39 b and a radially extending, fan-shaped contact plate 39 c. Contact plate 39 c may be divided from control arm 39 so long as contact plate 39 c rotates following control arm 39.

As best seen in FIG. 11, first arm 39 a extends horizontally and forms, at one end thereof, an engaging portion 39 a′ for paralleling control shaft 35. Second arm 39 b extends upwardly and forms, at one end thereof, an engaging portion 39 b′ for paralleling control shaft 35. Engaging portions 39 a′ and 39 b′ project in opposite directions from each other.

Engaging portion 39 b′ is connected to a groove 11 d provided in a side surface of movable swash plate 11. Groove 11 d is formed between a pair of engaging projections 11 e disposed on the side surface of movable swash plate 11 and are longitudinally spaced apart at a predetermined interval. In such a construction, when control arm 38 is rotated around an axis lateral to the vehicle body, resulting in being rotated longitudinally of the vehicle body, control arm 39 rotates longitudinally around control shaft 35 to enable movable swash plate 11 to be slantingly operated and the hydraulic pump to be operated to change the quantity and direction of its discharging oil.

As shown in FIGS. 2 and 10, a coiled neutral return spring 31 is fitted onto bush 51. Both ends of neutral return spring 31 are crossed to extend in the direction of first arm 39 a and sandwich between them an eccentric shaft 33. As shown in FIG. 1, eccentric shaft 33 is mounted to an inside wall of upper half housing 1 near control shaft 35 and engaging portion 39 a′ formed at one end of control arm 39.

Accordingly, when control lever 38 is turned to change the vehicle speed, control arm 39 is turned and one end of neutral return spring 31 is moved away from the other end, which is received by eccentric shaft 33, thereby applying a biasing force to control lever 38 to return it to the neutral position. When operating force to the speed changing member is released, a restoring force generated at one end of neutral return spring 31 returns engaging portion 39 a′ toward eccentric shaft 33 so as to hold control arm 38 in the neutral position. The extension of eccentric shaft 33 outside of the housing creates an adjusting screw. When the adjusting screw is loosened and eccentric shaft 33 is rotatably shifted, control arm 39 is shifted around control shaft 35 through neutral return spring 31 so that movable swash plate 11 can be adjusted to be in an accurate neutral position.

Contact plate or shifting member 39 c is fan-shaped around a center of curvature X so as to abut against pistons 64 along its entire rotational range between the furthest forward and furthest rearward positions, including a neutral position. As shown in FIGS. 10, 11 and 12, grooves 39 d are formed in contact plate 39 c at positions which abut against discharging oil holes 64 c of pistons 64 when control arm 39 is in the neutral position, and extend therefrom to the fringe of fan-shaped contact plate 39.

The interior of the closed fluid circuit is connected with the oil sump in the housing through grooves 39 d and discharging oil holes 64 c. When control arm 39 is rotated firm the neutral position so that movable swash plate 11 is slantingly rotated beyond a predetermined angle, discharging oil holes 64 c are cut off from contact with grooves 39 d. The surface of contact plate 39 c abutting oil holes 64 c at this time is smooth and plain. As shown in FIG. 2, a retaining plate 68 is disposed at a side of contact plate 39 c opposite to oil holes 64 c and is fixed to the inner portion of lower half housing 2. When pistons 64 are advanced by oil pressure, contact plate 39 c is sandwiched between pistons 64 and retaining plate 68 so as to be given a rotational resistance against the biasing force of neutral return spring 31.

Alternatively, contact plate 39 c may be disposed between pistons 64 and the inner wall of lower half housing 2, without a retaining plate 68 so as to provide rotational resistance directly by the inner wall of lower half housing 2. Further, rather than being screwed into bushes 65 in the open ends of first oil passage 5 a and second oil passage 5 b, pistons 64 can be directly, slidably inserted into the open ends of first oil passage 5 a and second oil passage 5 b, as shown in FIG. 13.

When control lever 38 is rotated by operating a speed changing member, control arm 39 is rotated by control shaft 35 so as to slantingly rotate movable swash plate 11 which is connected by engaging portion 39 b′ to engaging projections 11 e thereof, thereby changing the quantity of fluid discharged from the hydraulic pump. Accordingly, the rotational direction and speed of motor shaft 4 of the hydraulic motor is shifted to correspond with the rotational direction and degree of the speed changing member, so as to transmit driving force to axles 7.

In this case, whichever of first oil passage 5 a and second oil passage 5 b has higher pressure oil receives pressure in proportion to the load on axles 7 so that one of pistons 64 slides outwardly and pushes against contact plate 39 c of control arm 39. The friction force generated by such pushing is set to be smaller than the biasing force of neutral return spring 31. Therefore, an operator must operate the speed changing member with an operating force exceeding the friction force and the biasing force. After movable swash plate 11 is slantingly rotated beyond the predetermined position, discharging oil holes 64 c of pistons 64 are sealed by the smooth and plain surface of contact plate 39 c, whereby operating oil circulating in the closed fluid circuit does not leak therefrom so as to maintain the volume efficiency of the HST.

In such a condition, when the operator releases operating force applied to the speed changing member, control arm 39 is rotated toward the neutral position by the biasing force of neutral return spring 31. Pressure of piston 64 generates a friction force against contact plate 39 c of contact plate 39, as above mentioned, causing a resistant against the rotation toward the neutral position. Thereby, control arm 39 is gradually rotated toward the neutral position. As a result, a dynamic brake is not applied so that a vehicle does not stop suddenly.

When control arm 39 reaches the proximity of the neutral position, discharging oil holes 64 c of pistons 64 communicate with grooves 39 d so that the pushing force of pistons 64 against contact plate 39 c and any remaining pressure in the closed fluid circuit escapes, thereby moderating the braking shock and enlarging the range of the neutral position of the HST.

One of pistons 64 is disposed in the high-pressure oil passage for running the vehicle in a forward direction, the other is disposed in the high—pressure oil passage running the vehicle in a reverse direction. Both of the high-pressure oil passages, first oil passage 5 a and second oil passage 5 b, are separated from each other so that an operational condition of one piston 64 does not interfere with that of the other piston 64. Hence, the operational condition of each piston 64 can be adjusted individually so as to enable each of the friction forces against contact plate 39 c to meet the individual requirements to braking the vehicle when advancing and when backing the vehicle up. As a result, a vehicle can avoid stop shock when braking both in the case of an advancing vehicle and one that is backing up.

The operational condition of each piston 64 can be adjusted by modifying the diameter of discharging oil hole 64 c and/or the width or shape of groove 39 d. In FIG. 14 is disclosed an alternative embodiment of the present invention in which the shape of groove 39 d has been modified. In this embodiment, the depth of the grove varies in that groove 39 d is shallower the closer it is to communicating with discharging oil hole 64 c. This modification of groove 39 d can also be applied to the embodiment of the invention in which bush 65 is interposed between piston 64 and center section 5, as shown in FIG. 12.

As shown in FIGS. 8 and 9, the above mentioned center section 5 includes first oil passage 5 a and second oil passage 5 b in parallel to each other on a common horizontal plane. In FIGS. from 15-20, an alternative embodiment of the present invention will be described which comprises a center section 5′ instead of center section 5. In this embodiment, center section 5′ includes a horizontal first oil passage 5′a and a horizontal second oil passage 5′b disposed in parallel to each other along a common vertical plane, so that first and second oil passages 5′a and 5′b overlap with each other as seen in the cross-sectional plan view of FIG. 16. As best seen in FIG. 17, each of kidney ports 41′a and 41′b which are open at a motor mounting surface 41′ formed on the vertical portion 501′ of center section 5′, communicates with one end of each of first and second oil passages 5′a and 5′b, respectively.

Kidney ports 40′a and 40′b are open at a pump mounting surface 40′ formed on a horizontal portion 500′ of center section 5′. Kidney port 40′a is open above first oil passage 5′a and extends downwardly to directly communicate with oil passage 5′a. As best seen in FIG. 18, kidney port 40′b is above, but to one side of second oil passage 5′b and communicates with second oil passage 5′b through a connecting oil passage 40′c which is slantingly and downwardly disposed from the outside of center section 5′ to second oil passage 5′b. The outer open end of connecting oil passage 40′c is closed by a plug 69.

As seen in FIG. 17, the other ends of first and second oil passages 5′a and 5′b are open along an outside surface of horizontal portion 500′. A check valve 54 is disposed in each of the open ends of oil passages 5′a and 5′b for supplying oil to the closed fluid circuit. Each check valve 54 is closed by a plug 70. Each outer end of plug 70 abuts against a projection 71 formed along an inner surface of the housing to prevent the plugs 70 from slipping out.

A supply poll 5′g is open at the lower surface of horizontal portion 500′ and extends upwardly within center section 5′ communicating with the entrance ports of check valves 54 in both first and second oil passages 5′a and 5′b. Supply port 5′g opens within an oil filter 56 disposed between the bottom of center section 5′ and the bottom surface of lower half housing 2, as the previously described embodiments. Each of first and second oil passages 5′a and 5′b is supplied with oil from the housing which is filtered by oil filter 56 through supply port 5′g and check valves 54.

Oil holes 5′e and 5′f are horizontally branched from first and second oil passages 5 a′ and 5′b, respectively toward one side surface of center section 5′. The outer opening end of each of oil holes 5′e and 5′f is provided with a piston 64. In this embodiment, a flange and bush, as in the previous embodiments are not used.

Both oil holes 5′e and 5′f are disposed in parallel to each other with their axes along a common vertical plane so that pistons 64 are disposed in a vertical row, as shown in FIGS. 18 and 20. Accordingly, only one groove 39′d is formed on contact plate 39′ along a surface which abuts against pistons 64 and communicates with both discharging oil holes 64 c of pistons 64 simultaneously when contact plate 39′c is positioned in the neutral position. Because contact plate 39′c has only one groove 39′d, it can be more narrow as compared with contact plate 39 c of the previous embodiments so as to make the space around it more compact.

When it is desired to moderate the difference in braking shock caused by advancing and backing of the vehicle, the braking shocks may be individually moderated by modifying the diameter of discharging oil holes 64 c of pistons 64.

Center sections 5 and 5′ mentioned above, which form pump mounting surfaces 40 and 40′, respectively and motor mounting surfaces 41 and 41′, respectively which are perpendicular to each other, may be adapted to an HST having a pump shaft and a motor shaft which are disposed in parallel to each other. Such a center section is shown in FIGS. 21 and 22 in which center section 5″ forms a plate for an HST having a pump shaft 3 and a motor shaft 4 which are disposed in parallel to each other.

Center section 5″ is formed as a thin plate. Both a pump mounting surface 40″ and a motor mounting surface 41″ are formed along a the top surface hereof A first oil passage 5″a and a second oil passage 5″b are bored in center section 5″ below pump mounting surface 40″ and motor mounting surface 41″. First and second oil passages 5″a and 5″b are disposed in parallel to each other on a common horizontal plane. Kidney ports 40″a and 40″b are open at pump mounting surface 40″. Kidney ports 41″a and 41″b are open at motor mounting surface 41″. Kidney ports 40″a and 41″a extend downwardly and communicate with first oil passage 5″a. Kidney ports 40″b and 41″b extend downwardly and communicate with second oil passage 5″b.

An oil hole 5″c and an oil hole 5″d extend downwardly from first and second oil passages 5″a and 5″b, respectively. In the opening of each of oil holes 5″c and 5″d which are open at the bottom surface of center section 5″ is disposed a check valve 54 so as to enable operating oil to be supplied from the oil sump in the housing to each of first and second oil passages 5″a and 5″b.

One end of each of first and second oil passages 5″a and 5″b is open at one side surface of center section 5″ and is closed by plug 70. Each plug 70 abuts against a projection 71 formed at the interior of the housing. First and second oil passages 5″a and 5″b differ in length so that the other end of first passage 5″a is offset from the other end of second oil passage 5″b. Horizontal oil holes 5″e and 5″f are branched perpendicularly from approximate the inner ends of first and second oil passages 5″a and 5″b, respectively. Both oil holes 5″e and 5″f are open at another side surface of center section 5″. A piston 64, as discussed above, is disposed in each opening of oil holes 5″e and 5″f. At the exterior side of pistons 64 is disposed contact plate 39 having a pair of grooves 39 d, as discussed above.

Another embodiment of an axle driving apparatus will be described with reference to FIGS. 23, 24 and 25. The HST of the axle driving apparatus of previously described embodiments includes a pump shaft 3 and motor shaft 4 which are disposed perpendicular to each other and have a movable swash plate 11 of a cradle-type which is separated from a control arm 39 or 39′ for the hydraulic pump. The axle driving apparatus of this alternative embodiment has a center section 5″, as shown in FIGS. 21 and 22 for supporting pump shaft 3 and motor shaft 4 in parallel to each other and has a trunnion-type movable swash plate 11′ which forms a control shaft as single body. Center section 5″ in FIGS. 21 and 22 differs from that in FIG. 23 and 24 in, among other things, appearance and the shape of the oil holes, however, the technical idea of the latter center section 5″ is the same as that of the former.

With specific reference to FIGS. 23-25, center section 5″ is disposed within a lower half housing 2′. On pump mounting surface 40″ and motor mounting surface 41″ formed on the upper surface thereof, is mounted a cylinder block for the hydraulic pump and a cylinder block for the hydraulic motor, respectively, thereby constituting an HST. Pump shaft 3 is connected with cylinder block 16 of the hydraulic pump and is vertically disposed and rotatably supported through bearings 75 at the upper portion of upper half housing 1′. The lower end thereof is rotatably inserted into center section 5″. Movable swash plate 11′, which is of a trunnion-type, is disposed above cylinder block 16 in upper half housing 1′.

The entire movable swash plate 11′ is shown in FIG. 25. A pair of trunnion shafts 11′a and 11′b are formed on movable swash plate 11′ and project in opposite directions from both sides thereof. A fan-shaped contact plate 11′c, equivalent to contact plate 39 c, is formed below the base end of trunnion shaft 11′b, equivalent to control shaft 35. A pair of grooves 11′d, equivalent to grooves 39 d, are formed at an inner surface of contact plate 11′c.

In upper half housing 1′, trunnion shaft 11′a is supported by a side wall thereof through a bush 73. Trunnion shaft 11′b is supported by a lid 72 attached to upper half housing 1′ through an other bush 74. Control lever 38 is fixed to the exterior portion of trunnion shaft 11′b projecting from lid 72. The surface of contact plate 11′c forming the pair of grooves 11′d abuts against the utmost ends of pistons 64 inserted into center section 5″. Restraining plate 68 is interposed between the inner wall of lower half housing 2′ and contact plate 11′c. Such a construction constitutes a neutral return position member for returning the movable swash plate 11′ to the neutral position effecting the same as that comprising pistons 64 and contact plate 39 c, described above.

Next, an axle driving apparatus having center section 5′ as described in FIGS. 15 through 20 will be explained in accordance with FIGS. 26 through 58. The members whose construction and function are identical and similar with those shown in FIGS. 1 through 20 are marked with the same reference numerals and the detailed descriptions of them are hereinafter omitted.

As shown in FIGS. 26 through 28, center section 5′, which is substantially similar to that shown in FIGS. 15 through 20, is compactly disposed in first chamber R1 within the housing of the axle driving apparatus. Hydraulic pump 45 having vertical pump shaft 3 is disposed between axles 7 and hydraulic motor 46 having horizontal motor shaft 4 is disposed parallel to axles 7.

As shown in FIGS. 27 and 28 according to this embodiment, first chamber R1 and second chamber R2 are filled with oil so as to constitute an oil sump. Chambers R1 and R2 are connected with each other through a passage 9 so as to allow the inner oil of housing to interflow between chambers R1 and R2. Passage 9 is covered at the open top thereof with a lid 78 and provided on the bottom thereof with a seat 63. A discoid oil filter 77 is vertically disposed between lid 78 and seat 63 for removal of such impurities as metal fragments, which are generated by the rubbing of gears against each other, from oil in chambers R1 and R2.

Center section 5′ is provided therein with first and second oil passages 5′a and 5′b horizontally disposed in a vertical row as the above. Oil holes 5′e and 5′f horizontally branch in a vertical row from oil passages 5′a and 5′b, respectively, and are plugged at the outward open ends thereof with an upper piston 64U and a lower piston 64L, respectively. Pistons 64U and 64L constitute an internal damping system (IDS) which is provided mainly for moderating the dynamic braking shock during returning of the speed changing member to neutral and secondly for moderating the shock on starting of advancing and reversing. For this embodiment, as shown in FIG. 29, pistons 64U and 64L, which are bored therein by respective outward open upper and lower orifices 80U and 80L serving as the above-mentioned discharging oil holes 64 c, are slidably fitted in outward open cylindrical portions 5′h formed at the ends of oil holes 5′e and 5′f, respectively. Seal 66 is interposed between the wall of cylindrical portion 5′h and the periphery of each of pistons 64U and 64L for avoiding an oil leak. Coiled springs 79 are interposed between the inner ends of pistons 64U and 64L and the inner ends of cylindrical portions 5′h so as to bias pistons 64U and 64L outwardly. The biasing force of coiled spring 79 is set to be smaller than that of coiled and twisted neutral return spring 31 winding around control shaft 35.

Next, referring to FIGS. 30 through 62, explanation will be given on various embodiments and modifications regarding control arm 39′ with contact plate 39′c suiting pistons 64U and 64L fitted in center section 5′ applied in the embodiment shown in FIGS. 26 through 29.

A preferred first embodiment of control arm 39′, shown in FIGS. 30 and 31, has contact plate 39′c, which is provided on the surface thereof facing pistons 64U and 64L with vertical groove 39′d similar to that shown in FIG. 20. Vertical groove 39′d has a width which is larger than the diameters of orifices 80U and 80L. When control arm 39′ is positioned in neutral, orifices 80U and 80L are in communication with vertical groove 39′d, so that oil within the closed fluid circuit is released from orifices 80U and 80L through vertical groove 39′d into the oil sump formed in the housing of the axle driving apparatus. Thus, even if an error with respect to the neutral positioning of movable swash plate 11 occurs so that the closed fluid circuit is wrongly supplied with oil by hydraulic pump 45, hydraulic motor 46 is prevented from wrong slight rotation.

When the vehicle is left on a slope or slanted ground when the HST is in neutral and axles 7 are not locked for parking, the weight of vehicle is applied on axles 7 so as to rotate them in the descending direction. Then, the load on axles 7 generates a back-pressure in the closed circuit through hydraulic motor 46. In this case, if the inner oil of the closed circuit is drained through wide-open orifices 80U and 80L and vertical groove 39′d into the oil sump, such reduced hydraulic pressure cannot hold hydraulic motor 46, whereby hydraulic motor 46 freely follows the rotation of axles 7 and the vehicle descends the slope. However, each of orifices 80U and 80L of this embodiment is made smaller with its diameter than conventional one, whereby the oil cannot be drained therefrom perfectly. Hence, hydraulic motor 46 is held by the increased hydraulic pressure against the rotational force of axles 7 generated by the weight of vehicle, thereby enabling the vehicle to stay.

As shown in FIGS. 30 and 31, contact plate 39′c is provided with upper and lower first transverse grooves 39′g and 39′h and upper and lower second transverse grooves 39′i and 39′j branching transversely from vertical groove 39′d.

The relation of arrangement between first and second oil passages 5′a and 5′b may be reversed. However, this embodiment and others hereinafter will be described on the premise that first oil passage 5′a is disposed above second oil passage 5′b. It will be understood that in the rotational range of control arm 39′ for advancing, first oil passage 5′a is lower-pressured and second oil passage 5′ b is higher—pressured during acceleration, however, first oil passage 5′a becomes pressured higher than second oil passage 5′b because of back-pressure occurred by the rotation of hydraulic motor 46 following the rotation of axles 7 during deceleration by the returning of control arm 39′ to neutral. Similarly, in the rotational range of control arm 39′ for reversing, second oil passage 5′b, which is lower-pressured during acceleration, becomes pressured higher than first oil passage 5′a during the returning of control arm 39′ to neutral.

For this embodiment, upper first transverse groove 39′g having a slight range is disposed so as to communicate with upper orifice 80U of upper piston 64U connected to first oil passage 5′a when control arm 39′ is positioned in its rotational range for reversing (hereinafter, reversing range R). Lower first transverse groove 39′h having a slight range is disposed so as to communicate with lower orifice 80L of lower piston 64L connected to second oil passage 5′b when control arm 39′ is positioned in its rotational range for advancing (hereinafter, advancing range A). An angle α1 which the center line of the width of vertical groove 39′d passing center 0 of control shaft 35 (when control arm 39′ is in neutral position N, the center line is common with a neutral line NL passing center 0 and centers of orifices 80U and 80L) forms with the line passing the utmost end of upper first transverse groove 39′g and center 0 is as large as an angle α2 which the same center line forms with the line passing the utmost end of lower first transverse groove 39′h and center 0.

Angles α1 and α2 may be different. They can be optionally determined so as to enable the advancing and reversing vehicle to start without a shock.

On the start of advancing, the increase of hydraulic pressure closed fluid circuit within center section 5′ is slightly delayed because higher-pressured second oil passage 5′b is in communication with lower first transverse groove 39′h through lower orifice 80L, and after control arm 39′ is rotated so that lower orifice 80L passes the end of lower first transverse groove 39′h, the amount of oil supplied to hydraulic motor 46 by hydraulic pump 45 is increased so much as to enable the vehicle to start. On the start of reversing, similarly, the hydraulic pressure within the closed fluid circuit reaches the degree capable of driving hydraulic motor 46 after control arm 39′ is rotated so as to let upper orifice 80U pass the end of upper first transverse groove 39′g. Thus, the starts of advancing and reversing are comfortably moderated.

Upper second transverse groove 39′i having the predetermined range is disposed so as to communicate with upper orifice 80U of upper piston 64U connected to first oil passage 5′a when control arm 39′ is positioned in its advancing range A. Lower second transverse groove 39′j having the predetermined range is disposed so as to communicate with lower orifice 80L of lower piston 64L connected to second oil passage 5′b when control arm 39′ is positioned in its reversing range R. An angle of β1 which the center line of vertical groove 39′d passing center 0 of control shaft 35 forms with the line passing the utmost end of upper second transverse groove 39′i and center 0 is as large as an angle β2 which the same center line forms with the line passing the utmost end of lower second transverse groove 39′j and center 0.

Angles β1 and β2 may be different. They can be optionally determined so as to enable the advancing and reversing vehicle to stop without a braking shock.

When a speed changing member on the vehicle is released by an operator during advancing or reversing of the vehicle, control arm 39′ returns to neutral by biasing of neutral return spring 31. In case of the absence of second transverse grooves 39′i and 39′j so far, until control arm 39′ reaches its neutral position, a dynamic brake regarded as an engine brake is applied so as to decelerate the vehicle rapidly. When reaching the neutral position, the pressure oil in the closed fluid circuit is drained from both of orifices 80U and 80L to the oil sump through vertical groove 39′d, whereby the hydraulic pressure is reduced suddenly so that hydraulic motor 46 is made freely rotatable and axles 7 run idle. Accordingly, in case of deceleration in advancing, an operator leans forward until reaching neutral, and when reaching neutral, the decelerating force is suddenly lost, thereby making the operator bend backward. Thus, the operator is forced to change his/her posture greatly, and it will make him/her very uncomfortable.

In case of employing the above constructed contact plate 39′c having second transverse grooves 39′i and 39′j, when the speed changing member is released during advancing (or reversing), the increased pressure oil in one of oil passages 5′a (5′b), which is pressured higher by back-pressure generated from self-rotating of hydraulic motor 46 while it is set to be lower-pressured by movable swash plate 11, is drained from corresponding orifice 80U (80L) to the oil sump through corresponding second transverse grooves 39′i (39′j) for a short time before returning control arm 39′ reaches its neutral position N. Hence, the back pressure in the closed circuit is reduced until it reaches the peak thereof, thereby enabling the vehicle to stop smoothly.

The hydraulic action and its effect on the movement of vehicle in r elation to the whole rotational positions of control arm 39′ will be described according to FIGS. 32, 33 and 35.

In FIG. 35, “OF” designates the condition that orifice 80U or 80L is fully open toward the oil sump when it is located in one of grooves 39″d, 39′g, 39′h, 39′i and 39′j. “PO” designates that orifice 80U or 80L is partly open toward the oil sump when it is put on the end of one of first and second transverse grooves 39′g, 39′h, 39′i and 39′j so as to be partly shut by contact plate 39′c. And “S” designates that orifice 80U or 80L is completely shut by contact plate 39′c. The same designations are used in FIGS. 38, 41, 44, 46, 48, 51, 53, 55 and 58.

The attitudes of control arm 39′ in the rotational movement thereof marked as (a)-(g) shown in FIG. 32, 33 and 35 will be employed in similarly in all embodiments of control arm 39′ hereinafter.

Referring to FIGS. 32 and 35, the hydraulic action and its effect on the movement of vehicle in the advancing range A of control arm 39′ will be described.

When an operator operates the speed changing member provided on the vehicle for acceleration in advancing so as to rotate control lever 38 of the axle driving apparatus from its neutral position into its advancing range, control arm 39′ is rotated together with control lever 38 and control shaft 35 so as to change its position from (a) to (d) through (b) and (c) in FIG. 32.

During the rotation of control aim 39′, movable swash plate 11 of hydraulic pump 45 is shifted so as to accelerate the advancing rotation of hydraulic motor 46 with motor shaft 4, thereby accelerating the advancing rotation of axles 7. Higher-pressured second oil passage 5′b is back-pressured further in proportion to the load applied on axles 7, thereby making lower piston 64L slide outwardly and press against contact plate 39′c. The friction force between friction plate 68 and contact plate 39′c pressed there against through lower piston 64L by the maximum hydraulic pressure and the biasing force of coiled spring 79 is set to be smaller than the biasing force of neutral return spring 31, so that control arm 39′c automatically returns to neutral when releasing the speed changing member. For acceleration of the advancing speed, an operator must apply an operating force which exceeds the amount of such friction force and the biasing force of neutral return spring 31, onto the speed changing member.

At the position (a), both of orifices 80U and 80L are in communication with vertical groove 39′d, so that even if hydraulic pump 45 wrongly discharges oil into the closed circuit by an error of connecting means or linkage so as to apply hydraulic pressure to either oil passage 5′a or 5′b higher than the predetermined, the increased pressure oil therein. is drained to the oil sump through either of orifices 80U and 80L. Also, when the vehicle is left in neutral on a slope so that hydraulic motor 46 is wrongly driven so as to apply back-pressure to either oil passage 5′a or 5′b, the increased pressure oil therein cannot be drained perfectly therefrom because orifices 80U and 80L have such small diameters as described above, thereby preventing the vehicle from suddenly descending the slope.

During the rotation of control arm 39′ from the position (a) to the position (b), second oil passage 5′b is essentially higher-pressured slightly by hydraulic pump 45, however, it is the fact that the hydraulic pressure therein tends to become higher than the predetermined base on the position of movable swash plate because of the load of staying axles 7 thereby causing the vehicle to start suddenly. According to this embodiment, lower orifice 80L enters lower first transverse groove 39′h, so that part of pressure oil in second oil passage 5′b by the back-pressure is drained, thereby moderating the starting shock. In addition, upper orifice 80U in communication with lower-pressured first oil passage 5′a enters upper second transverse groove 39′i.

When passing the position (c), lower orifice 80L passes the end of lower first transverse groove 39′h, so that lower piston 64L pushes contact plate 39′c to shut lower orifice 80L, whereby higher-pressured second oil passage 5′b is increased in its hydraulic pressure. However, upper orifice 80U is still open to second transverse groove 39′i.

When reaching the position (d), upper orifice 80U also passes the end of upper second transverse groove 39′i so as to be shut by contact plate 39′c, thereby enabling the entire capacity of the HST to perform advancing acceleration. In this state, contact plate 39′c is slidably rotated against friction plate 68, thereby preventing control arm 39′ from rapid rotation. Hence, the vehicle is prevented from a rapid acceleration.

In case that an operator releases the operating force applied on the speed changing member during advancing so as to automatically rotate control arm 39′ from the position (d) to the position (a) through the positions (c) and (b), in other words, in case of returning to neutral, the rotation of axles 7, whose force exceeds that of motor shaft 4, drives hydraulic motor 46 so as to function as a hydraulic pump, whereby lower-pressured first oil passage 5′a is changed to be pressured higher than second oil passage 5′b. In this condition, after control arm 39′ passes the position (c), the excessive pressure oil in first oil passage 5′a is drained through upper orifice 80U and upper second transverse groove 39′i, thereby preventing the vehicle from sudden deceleration. Thus, the vehicle is moderated in its deceleration and stopping.

Referring to FIGS. 33 and 35, the hydraulic action and its effect on the movement of vehicle in the reversing range R of control arm 39′ will be described. In case of reversing acceleration from neutral, an operator shifts the speed changing member so as to rotate control arm 39′ from the position (a) to the position (g) through the positions (e) and (f). On starting, control arm 39′ is positioned at the position (e), part of oil in higher-pressured first oil passage 5′a, which is increased by hydraulic pump 45 and the load of axles 7, is drained through upper orifice 80U and first transverse groove 39′g, thereby preventing the vehicle from sudden start. In case of reversing deceleration by releasing the speed changing member so as to rotate control arm 39′ by biasing of neutral return spring 31 from the position (g) to the position (a) through the positions (f) and (e), lower orifice 80L enters lower second transverse groove 39′j after control aim 39′ passes the position (f), so that the excessive pressure oil in second oil passage 5′b, which is pressured higher by the rotational force of axles 7, is drained, thereby preventing the vehicle from sudden deceleration and rapid stopping.

The first embodiment of control arm 39′ (including the following modifications thereof) comprising contact plate 39′c, vertical grooves 39′d and second transverse grooves 39′i and 39′j between the position (c) and the position (f) through the position (a) as neutral position N constitutes the valves shown in FIG. 34 which control the oil releasing of orifices 80U and 80L. In FIG. 34, first transverse grooves 39′g and 39′h are out of consideration. The orifice opening and closing action of control arm 39′ is controlled together with movable swash plate 11 of hydraulic pump 45 by operation of speed changing member and biasing of neutral return spring 31. According to the opening and closing action, both upper and lower orifices 80U and 80L are fully open to the oil sump at neutral position N. In advancing range A, upper orifice 80U is opened to drain the pressure oil from first oil passage 5′a and lower orifice 80L is closed. In reversing range B, lower orifice 80L is opened to drain the pressure oil from second oil passage 5′b and upper orifice 80U is closed.

Next, various modifications of the first embodiment of control arm 39′ as shown in FIGS. 30 and 31 will be described according to FIGS. 36 through 41.

A first modification thereof shown in FIG. 36, in which first transverse grooves 39′g and 39′h are eliminated, is adapted to such case that the moderation of starting shock does not have to be considered. In FIG. 35, “PO” of upper orifice 80U in the position (e) and lower orifice 80L in the position (b) are changed into “S”.

With regard to a second modification shown in FIG. 37, vertical groove 39′d and first transverse grooves 39′g and 39′h are similar to the preferred first embodiment shown in FIGS. 30 and 31. One of second transverse grooves 39′i and 39′j is extended so that either orifice 80U or 80L is in communication with the extended second transverse groove at any position of one of advancing and reversing ranges A and R of control arm 39′. In the case shown in FIG. 38, an extending groove 39′n is formed so as to extend from the end of lower second transverse groove 39′j, whereby lower orifice 80L is in communication with either 39′j or 39′n at all the positions between the neutral position N and the maximum position of reversing range R of control arm 39′. Lower second transverse groove 39′j and extending groove 39′n are formed in a bending shape corresponding to the locus of lower orifice 80L. They may be formed in an arcuate shape. Accordingly, when the speed changing member in the reversing range is released, the excessive pressure oil in second oil passage 5′b is drained through lower orifice 80L and the series of extending groove 39′n and lower second transverse groove 39′j in all the reversing range R of control arm 39′, so that dynamic braking is rarely applied during deceleration of reversing between the maximum position and the neutral position. Such variation of opening condition of orifices 80U and 80L in relation to the whole rotational positions of control arm 39′ according to the second modification of the first embodiment is shown in FIG. 38.

With regard to a third modification shown in FIG. 39 serving as a modification of the second modification shown in FIG. 37, extending grooves 39′m and 39′n are extended from the ends of both second transverse grooves 39′i and 39′j, respectively, so that the two series of grooves 39′i and 39′m, and 39′j and 39′n form a bending shape corresponding to the locus of orifices 80U and 80L in all of the advancing and reversing rotational ranges A and R of control arm 39′. Thus, even in the event of returning to neutral from the maximum position of advancing and reversing of the speed changing member, the excessive pressure oil in the higher-pressured one of oil passages 5′a and 5′b is drained until the neutral position, thereby preventing the vehicle from dynamic braking during deceleration of advancing and reversing.

With regard to a fourth modification shown in FIG. 40, which is a modification of that shown in FIG. 39, the cross-sectional areas of second transverse grooves 39′i and 39′j and extending grooves 39′m and 39′n are large enough to drain the excessive pressure oil in the higher-pressured one of oil passages 5′a and 5′b sufficiently, thereby allowing vertical groove 39′d to be removed. Hence, control arm 39′ becomes so simple as to be produced at low cost.

Such variation of conditions of orifices 80U and 80L in relation to the whole rotational positions of control arm 39′ according to the third and fourth modifications of the first embodiment is shown in FIG. 41.

Next, explanation will be given on a second embodiment of control arm 39′ in accordance with FIGS. 42 through 48. As shown in FIG. 43, 44, 46 and 48, the second embodiment of control arm 39′ is characterized in that both orifices 80U and 80L are partly open toward the oil sump in neutral position N.

A preferred second embodiment of control arm 39′ shown in FIG. 42 is provided on the surface of contact plate 39′c thereof with upper and lower second transverse grooves 39′i and 39′j which have the construction and function similar to the first embodiment. Vertical groove 39′d of the first embodiment is replaced with upper vertical groove 39′d′ and lower vertical groove 39′d″, which are connected with upper second transverse groove 39′i and lower second transverse groove 39′j, respectively, and are open at the upper and lower ends thereof toward the oil sump. Orifices 80U and 80L are diametrically larger than those of the first embodiment, so that when the vehicle in neutral is left on a slope, the increased pressure oil by back pressure is drained so much as to become impossible to hold hydraulic motor 46. It will be advantageous in manufacturing and will prevent orifices 80U and 80L, which are diametrically larger, from stop up.

Furthermore, the center line of the width of upper vertical groove 39′d′ is offset from the vertical line serving as the horizontal center of contact plate 39′c, which is common with neutral line NL in neutral position N, toward upper second transverse groove 39′d, and that of lower vertical groove 39′i″ is offset therefrom toward lower second transverse groove 39′j, so that when control arm 39′ is at neutral position N, upper orifice 80U is put on a vertical edge of upper vertical groove 39′d′ in opposite to upper second transverse groove 39′i, and lower orifice 80L is put on that of lower vertical groove 39′d″ in opposite to lower second transverse groove 39′j. Thus, at the position (a) as neutral position N shown in FIGS. 43 and 44, the openings of orifices 80U and 80L are partly shut by contact plate 39′c, so as to be tightened. According to such construction, when the vehicle in neutral is left on a slope, the increased pressure oil by back pressure in the closed fluid circuit is hard to be drained, thereby preventing the vehicle from easily descending the slope.

At the positions (b) and (e) of control arm 39′ shown in FIG. 44 as the starting of advancing and reversing, one of orifices 80U and 80L is fully open to corresponding one of grooves 39′d, 39′d″, 39′i and 39′j, and the other is shut by contact plate 39′c, thereby enabling the vehicle to start for advancing and reversing swiftly.

At the positions (c) and (f) shown in FIG. 44, one of orifices 80U and 80L is put on the end of corresponding second transverse groove 39′i or 39′j so that the opening is tightened. During deceleration of advancing and reversing by returning to neutral, when control arm 39′ passes the positions (c) or (f), the excessive pressure oil in the higher-pressured oil passage 5′a or 5′b is drained through corresponding orifice 80U or 80L and second transverse groove 39′i or 39′j, thereby moderating the decelerating shook. At the positions (d) and (g), both orifices 80U and 80L are shut so that the HST can be operated with the whole of its capacity.

Referring to FIGS. 45 through 48, modifications of the second embodiment of control arm 39′ will be described.

With regard to a first modification shown in FIG. 45, lower vertical groove 39′d″ is removed. Extending groove 39′n is extended from the end of lower second transverse groove 39′j, whereby lower orifices 80L is fully open to the series of grooves 39′j and 39′n during the whole reversing range R as shown in FIG. 46, so that the excessive pressure oil in higher-pressured second oil passage 5′b is drained so as to moderate the dynamic brake during deceleration of advancing. The opening patterns of orifices 80U and 80L in relation to the whole rotational positions of control arm 39′ of the first modification is shown in FIG. 46.

With regard to a second modification shown in FIG. 47, upper vertical groove 39′d′ is also removed and extending groove 39′m is also extended from the end of upper second transverse groove 39′i in addition to the construction of the first modification. Due to this construction, upper and lower orifices 80U and 80L are fully open to the respective series of grooves 39′i and 39′m, and 39′j and 39′n during the whole advancing and reversing ranges A and R as shown in FIG. 48, so that the excessive pressure oil in higher-pressured one of oil passages 5′a and 5′b is drained to moderate the dynamic brake during deceleration of advancing and reversing.

Next, explanation will be given on a third embodiment of control arm 39′ in accordance with FIGS. 49 through 55. The third embodiment is characterized in that both orifices 80U and 80L are shut by contact plate 39′c when control arm 39′ is located in neutral position N or the position (a) as shown in FIGS. 50, 51, 53 and 55.

A preferred third embodiment of control arm 39′ shown in FIG. 49 is provided with second transverse grooves 39′i and 39′j formed similar to those of the first and second embodiments of control arm 39′. Upper vertical groove 39′d′, whose top is open to the oil sump, is offset from the vertical line constituting a horizontal center of contact plate 39′c, which is common with neutral line NL passing center 0 and both the centers of orifices 80U and 80L in neutral position N, so as to be connected at the lower end thereof with the intermediate portion of upper second transverse groove 39′i. Lower vertical groove 39′d″, whose lower end is open to the oil sump, is offset from the same vertical line so as to be connected at the upper end thereof with the intermediate portion of lower second transverse groove 39′j. At the position (a) of control arm 39′ as neutral position N shown in FIGS. 50 and 51, both orifices 80U and 80L are shut by contact plate 39′c, however, an orifice 81 is interposed between first and second oil passages 5′a and 5′b in the closed fluid circuit so as to be disposed in parallel to check valve 54 as shown in FIG. 50, thereby giving a dead zone of the HST in the neutral position thereof. The pressure oil, which is drained through orifice 81 at the neutral position, is so limited as to prevent the vehicle in neutral left on a slope from suddenly descending.

As shown in FIG. 51, in case of acceleration of advancing, at the position (b) of control arm 39′, lower orifice 80L connected to higher-pressured second oil passage 5′b is shut by contact plate 39′c, thereby enabling the vehicle to start swiftly.

In case of deceleration of advancing by returning to neutral, at the position (d) of control arm 39′, the closed fluid circuit is back pressured so that a dynamic brake is applied on the vehicle. At the position (c), upper orifice 80U connected to higher-pressured first oil passage 5′a is put on the end of upper second transverse groove 39′i to be tightened with its opening and afterward enters it, thereby moderating the dynamic brake. Just before the neutral position as the position (b), upper orifice 80U is tightened and is finally shut by contact plate 39′c at the neutral position (a), thereby applying the dynamic brake on the vehicle again so as to stop it.

In case of acceleration of reversing, at the position (e) of control arm 39′, upper orifice 80U connected to higher-pressured first oil passage 5′a is shut by contact plate 39′c, thereby enabling the vehicle to start swiftly.

In case of deceleration of reversing by returning to neutral, at the position (g) of control arm 39′c, the closed fluid circuit is back pressured so that a dynamic brake is applied on the vehicle. At the position (f), lower orifice 80L connected to higher-pressured second oil passage 5′b is put on the end of lower second transverse groove 39′i to be tightened with its opening and afterward enters it, thereby moderating the dynamic brake. Just before the neutral position as the position (e), lower orifice 80L is tightened and is finally shut by contact plate 39′c at the neutral position (a), thereby applying the dynamic brake on the vehicle again so as to stop it.

Referring to FIGS. 52 through 55, modification of the third embodiment of control arm 39′ will be described.

For a first modification shown in FIG. 52, lower vertical groove 39′d″ is removed and extending groove 39′n is extended from the end of lower second transverse groove 39′j. For a second modification shown in FIG. 54, lower vertical groove 39′d″ is similarly removed and extending groove 39′m is additionally extended from the end of upper second transverse groove 38′i.

According to the first modification, the dynamic brake is moderated during deceleration of advancing by returning to neutral, and at the neutral position, dynamic brake is applied so as to stop the vehicle. According to the second modification, the same phenomenon also occurs during deceleration of advancing by returning to neutral.

Next, explanation will be given on a fourth embodiment of control arm 39′ in accordance with FIGS. 56 through 58. This is a mixture of first and third embodiments. In this regard, as shown in FIG. 56, vertical groove 39′d is divided into upper vertical groove 39′d′ and lower vertical groove 39′d″, which are open to the oil sump. Similar to the first embodiment, lower vertical groove 39′d″, which is located along neutral line NL in neutral position N, is connected with lower first and second transverse grooves 39′h and 39′j. Similar to the third embodiment, upper vertical groove 39′d′ is offset so as to be connected with the intermediate portion of upper second transverse groove 39′i, so that it does not communicate with upper orifice 80U at neutral position N.

Due to such construction, as shown in FIGS. 57 and 58, at the position (a) as neutral position N, only lower orifice 80L is fully open to the oil sump and upper orifice 80U is shut by contact plate 39′c, so as to limit the drained excessive pressure oil in the closed circuit in neutral position N. Hence, the vehicle in neutral left on a slope is prevented from descending.

At the position (b) as starting of advancing, the excessive pressure oil in higher-pressured second oil passage 5′b is drained through lower orifice 80L which is fully open to first transverse groove 39′h and lower vertical groove 39′d″, thereby moderating the advancing starting shock. In this embodiment, moderation of the shock of reversing start is out of consideration so that upper first transverse groove 39′g is eliminated.

For a short time until reaching the neutral position during returning to neutral in advancing and reversing, one of orifices 80U and 80L enters corresponding second transverse groove 39′i or 39′j, thereby moderating the shock of deceleration and braking. Second transverse grooves 39′i or 39′j may be extended so as to form extending grooves 39′m or 39′n as the above. In this case, for all the time of returning to neutral in advancing or reversing, dynamic braking can be moderated.

As the fourth embodiment, the shape of groove on control arm 39′ can be constructed by mixing any two of the above three embodiments. Also, it can be constructed by employing any two of the above various modifications. Accordingly, the orifices 80U and 80L can be made in communication with the oil sump in various timing patterns.

Next, description will be given on a fifth embodiment of control arm 39′ in accordance with FIGS. 59 through 62. This embodiment does not provide an oil draining to the oil sump in the housing, which causes the problem that either orifice 80U or 80L which is negatively pressurized badly absorbs oil with air bubbles from the oil sump in the housing.

For details of the problem referring to the first embodiment, when control arm 39′ (movable swash plate 11) reaches the vicinity of its neutral Position so as to make orifices 80U and 80L open to the oil sump in the housing through grooves, pressure oil is drained from either orifice 80U or 80L of higher pressurized oil passage 5′a or 5′b. However, as oil drains from the orifice, the other orifice of negatively pressurized oil passage 5′b or 5′a absorbs oil with air bubbles from the oil sump through the corresponding groove. Then, the mixing of air bubbles into the oil circulated between both oil passages 5′a and 5′b brings the vehicle into a freewheeling condition, where the vehicle unexpectedly descends a slope by inertia.

Referring to the fifth embodiment for solving the problem, as shown in FIGS. 59 and 61, on the surface of contact plate 39′c abutting pistons 64U and 64L are opened a hole 39′x and a groove 39′i′ separated from each other in correspondence to upper orifice 80U, and a hole 39′y and a groove 39′j′ separated from each other in correspondence to lower orifice 80L, The distances between hole 39′x and groove 39′i′ and between hole 39′y and groove 39′j′ may be reduced as required. Holes 39′x and 39′y, which have a larger diameter than orifices 80U and 80L, are bored perpendicular to the surface of contact plate 39′c. The openings of holes 39′x and 39′y are positioned so as to communicate with orifices 80U and 80L when control arm 39′ (movable swash plate 11) is at the neutral position or its vicinity. A connection hole 39′z is bored in control arm 39′ in parallel to the surface of contact plate 39′c so as to join with holes 39′x and 39′y. Connection hole 39′z is drilled from the tail end of contact plate 39′c and its opening is plugged with a cap 90, thereby preventing oil leak from connection hole 39′z to the oil sump in the housing.

Grooves 39′i′ and 39′j′ are provided for moderation of dynamic brake. similar to grooves 39′i and 39′j. However, groove 39′i′ is spaced from hole 39′x at such a distance as to make orifice 80U partly open during the relative motion of orifice 80U between hole 39′x and groove 39′i′. The distance may be determined so as to close orifice 80U entirely for an extremely short time while moving between hole 39′x and groove 39′i′. Similarly, groove 39′j′ and hole 39′y are spaced from each other.

When control arm 39′ (movable swash plate 11) is located at the neutral position and its vicinity so as to make orifices 80U and 80L communicate with respective holes 39′x and 39′y, oil is drained from either orifice 80U or 80L of either oil passage 5′a or 5′b, which has been higher pressurized, into connection hole 39′z. and absorbed into the other negative pressurized oil passage 5′b or 5′a through the other orifice 80L or 80U. The oil circuit consisting of holes 39′x, 39′y and 39′z is closed so as to be hydraulically tightened, whereby oil passage 5′b or 5′a is fed with only the oil circulated through within center section 5′, control arm 39′ and corresponding piston 64L or 64U without the oil mixed with air bubbles in the housing. As a result, both oil passages 5′a and 5′b, when movable swash plate 11 and control arm 39′ are located at their neutral position, are prevented from the entry of air bubbles, so as to bring the vehicle safe from the freewheeling condition evenly pressurized so as to set the HST in neutral. thereby stopping the vehicle certainly.

Referring to FIG. 62, both orifices 80U and 80L are fully open to respective holes 39′x and 39′y when control arm 39′ (movable swash plate 11) is at the neutral position and within a certain advancing and reversing ranges from the neutral position. Since the rotational speed of lower orifice 80L which is further from control shaft 35 as a pivot than upper orifice 80U, is greater than that of orifice 80U while holes 39′x and 39′y are of the same diameter, the full open range of control arm 39′ including the neutral position for orifice 80L is narrower than that for orifice 80U. Alternatively, the diameter of hole 39′y may be greater than that of hole 39′x, thereby extending the width of full open range for orifice 80L to coincide with that for orifice 80U.

While orifice 80U relatively moves from hole 39′x to groove 39′i′ by rotation of control arm 39′ for advancing acceleration, orifice 80U is partly open to hole 39′x or groove 39′i′, and then, fully open to groove 39′i′. At last, orifice 80U passes groove 39′i′ and is shut by contact plate 39′c. Simultaneously, orifice 80L leaves hole 39′y, and then, becomes shut by contact plate 39′c. When control arm 39′ is rotated for advancing deceleration to the neutral position, oil passage 5′a becomes higher pressurized so that excessive pressure oil is drained from orifice 80U to groove 39′i′, thereby moderating a dynamic brake. When both holes 39′x and 39′y reach respective orifices 80U and 80L, both oil passages 5 a and 5 b are evenly pressurized, thereby stopping the vehicle.

Orifices 80U and 80L are similarly opened and closed in relation to holes 39′x and 39′y and groove 39′j′ so as to give the similar effect during the rotation of control arm 39′ for acceleration and deceleration of reversing.

Alternatively, for the purpose of preventing the negatively pressurized oil circuit in center section 5′ from air bubbles generated from the oil sump, pistons 64U and 64L having respective orifices 80U and 80L may be further provided with check valves which allow only flow of oil therethrough from oil passages 5′a and 5′b to orifices 80U and 80L, instead of modifying control arm 39′ as described above in the fifth embodiment. Description will now be given to embodiments of pistons 64U and 64L with check valves according to FIGS. 63-67.

Referring to FIG. 63, upper and lower plate-type check valves 92U and 92L are disposed in respective upper and lower cylinder holes 5′h. Each of check valves 92U and 92L (check valve 92) comprises a valve casing 93 and a discoid valve plate 94 disposed in valve casing 93. In cylinder hole 5′h, a compressed spring 91 is interposed between valve casing 93 and each of pistons 64U and 64L, so as to press each piston 64U or 64L against contact plate 39′c of control arm 39′ and press valve casing 93 against the internal end surface of cylinder hole 5′h. Valve casing 93 is provided at both opposite sides thereof with a first opening 93 a and a second opening 93 b, respectively. First opening 93 a, which is open toward corresponding oil passage 5′a or 5′b, is diametrically smaller than valve plate 94. Second opening 93 b, which is open toward corresponding orifice 90U or 80L, is diametrically larger than valve plate 94. As shown in FIG. 66, nails 93 c formed of valve casing 93 are disposed across second opening 93 b so as to prevent valve plate 94 from escaping through second opening 93 b.

Referring to FIG. 64, upper and lower ball-type check valves 95U and 95L are disposed in respective upper and lower cylinder holes 5′h. Each of check valves 95U and 95L (check valve 95) comprises a valve casing 96 and a valve ball 97 disposed in valve casing 96. In cylinder 5′h, a compressed spring 91 is interposed between valve casing 96 and each of pistons 64U and 64L, so as to press each piston 64U or 64L against contact plate 39′c of control arm 39 and press valve casing 96 against the internal end surface of cylinder 5′h. Valve casing 96 is provided at both opposite sides thereof with a first opening 96 a and a second opening 96 b, respectively. First opening 96 a, which is open toward corresponding oil passage 5′a or 5′b, is diametrically smaller than valve ball 97. Second opening 93 b, which is open toward corresponding orifice 80U or 80L, is diametrically larger than valve ball 97. As shown in FIG. 67, nails 96 c formed of valve casing 96 are disposed across second opening 96 b so as to prevent valve ball 97 from escaping through second opening 96 b.

Therefore, either check valve 92 or 95 allows oil to flow only from either oil passage 5′a or 5′b to either orifice 80U or 80L.

FIG. 65 shows ball-type check valves 95U and 95L are disposed in respective pistons 64U and 64L. Compressed spring 91 abuts at one end thereof against the inside end surface of cylinder hole 5′h toward each of oil passages 5′a and 5′b, and is inserted into each of pistons 64U and 64L so as to abut at the other end thereof against valve casing 96 of check valve 95, thereby biasing valve casing 96 together with each of pistons 64U and 64L so as to press each of pistons 64U and 64L against contact plate 39′c of control arm 39′. The location of check valve 95 in each of pistons 64U and 64L is advantageous in shortening of cylinder hole 5′h thereby compacting center section 5′. Alternatively, in pistons 64U and 64L may be disposed plate-type check valves 92U and 92L replacing ball-type check valves 95U and 95L.

Each of FIGS. 63, 64 and 65 shows such a condition that control arm 39′ is located at its neutral position so as to bring both orifices 80U and 80L into communication with the oil sump in the housing of the axle driving apparatus through oil groove 39′d, lower oil passage 5′b is hydraulically pressured higher than the predetermined pressure and upper oil passage 5′a is negatively pressured in relation to the oil sump. Referring to FIG. 63, valve plate 94 of lower check valve 92L is pressed against nails 93 c by the high oil pressure generated from lower oil passage 5′b, so that excessive oil from oil passage 5′b passes valve casing 93 of lower check valve 92L from first opening 93 a to second opening 93 b, and is drained into the oil sump through lower orifice 80L and oil groove 39′d. While the inside pressure of upper piston 64U is higher than upper oil passage 5′a, valve plate 94 and valve casing 93 of upper check valve 92U abut against each other so as to close first opening 93 a because of the difference of hydraulic pressure between the inside of piston 64U and oil passage 5′a, so that the oil circuits in center section 5′ is prevented from mixture of oil and air bubbles generated from the oil sump. Similarly, referring to each of FIGS. 64 and 65, lower check valve 95L allows excessive oil to flow from lower oil passage 5 b to the oil sump, and upper check valve 95U stops the oil flow from the oil sump into upper oil passage 5′a.

In the case that the hydraulic pressure of upper oil passage 5′a is higher than that of lower oil passage 5′b, the opposite occurs. That is, upper check valve 92U of FIG. 63 and 95U of FIGS. 64 and 65 will open to allow excessive oil flow from upper oil passage 5′a to the oil sump, and lower check valve 92L of FIG. 63 and 95L of FIGS. 64 and 65 prevent the flow of oil for the oil sump into lower oil passages 5′b.

Thus, since the oil circuits in center section 5′ are prevented from air bubbles generated in the oil sump in the housing of the axle driving apparatus due to check valves assembled with respective pistons 64U and 64L, the vehicle is safe from an unexpected freewheeling condition so that the vehicle, while being left on a slope in neutral without a brake, is certainly kept standstill.

An alternative construction shown in FIGS. 68 to 74 may be provided to attain the object which is the same as that of the fifth embodiment, that is, prevention of oil having air bubbles in the oil sump of the housing from being absorbed into depressed one of the orifices 8OU and 80L when control arm 39′ is operated for deceleration and reaches the neutral position. Description will be given on the alternative construction serving as a sixth embodiment of the present invention as follows: in the sixth embodiment, as shown in FIG. 68, upper and lower pistons 64U and 64L are bored at their top surfaces with oil supply/discharge ports 82U and 82L, respectively. The diameter of oil supply/discharge ports 82U and 82L are not so small as orifice 80 but are sufficiently large, for example, as large as branching oil passages 5′e and 5′f.

Contact plate 39′c is drilled out between its front and back surfaces in perpendicular to the surfaces with a pair of upper and lower oil through-passages 98U and 98L corresponding to oil supply/discharge ports 82U and 82L. At the back surface of contact plate 39′c abutting against a friction plate 68′, rear open ends of oil through-passages 98U and 98L face friction plate 68′. At the front surface of contact plate 39′c abutting against pistons 64U and 64L, front open ends of oil through-passages 98U and 98L, which are made diametrically slightly larger, are brought into communication with oil supply/discharge ports 82U and 82L when control arm 39′ is located at the neutral position or its vicinity. Upper and lower oil through-passages 98U and 98L are juxtaposed approximately in a line radially relative to the pivot of control arm 39′.

Control plate 39′c of this embodiment is not provided on its front surface with grooves 39′i′ and 39′j′ which are formed on that of the fifth embodiment.

Contact plate 39′c is drilled thereinto from its bottom end in parallel to its front and back surfaces so as to form a connection hole 101. Connection hole 101 crosses lower oil through-passage 98L perpendicularly and reaches upper oil through-passage 98U. The open bottom end portion of connection hole 101 is diametrically enlarged and formed into a female screw. Connection hole 101 has a diametric difference between upper and lower oil through-passages 98U and 98L so as to form a step 102. A cylindrical orifice member 100, having a closed end and an open end in opposite, is inserted into connection hole 101 from the bottom open end thereof, so that the closed end of orifice member 100 comes to abut against step 102. A cap 103 which is threaded in correspondence to the female screw of connection hole 101 plugs the bottom open end of connection hole 101 so as to close the open end of orifice member 100. Orifice member 100 is provided at its closed end surface with a pin hole 100 a serving as an orifice, which is open to the diametrically small portion of connection hole 101 above step 102 so as to communicate the inner space of orifice member 100 with upper oil through-passage 98U. Orifice member 100 is also provided at its round surface with a plurality of slits 100 b so as to communicate the inner space of orifice member 100 with lower oil through-passage 98L.

Friction plate 68′ abutting against the back surface of contact plate 39′c is formed with a pair of upper and lower recesses 68′a and 68′b corresponding to respective upper and lower pistons 64U and 64L. Friction plate 68′ forms a spring fixture portion 68′c between both recesses 68′a and 68′b. As shown in FIG. 71, a rectangular plate spring 104 is pierced at its central portion with a bolt 105, and bolt 105 together with plate spring 104 is screwed into a bolt hole 68′d drilled in spring fixture portion 68′c of friction plate 68′, so that plate spring 104 comes to cover upper and lower recesses 68′a and 68′b.

In friction plate 68′, upper and lower recesses 68′a and 68′b and spring fixture portion 68′c have a common width which is as large as that of plate spring 104. The surface of spring fixture portion 68′c is recessed to the depth which is as large as the thickness of plate spring 104, whereby the frictional surface of friction plate 68′ and the surface of plate spring 104 are arranged on a common flat surface in a contact with contact plate 39′c.

When control arm 39′ comes to the neutral position or its vicinity so as to bring oil supply/discharge ports 82U and 82L of pistons 64U and 64L into communication with respective oil through-passages 98U and 98L of contact plate 39′c, the rear openings of oil supply/discharge ports 98U and 98L at the back surface of contact plate 39′c are brought into contact with plate spring 104, thereby being covered with plate spring 104. If the hydraulic pressure of either oil through-passage 98U or 98L is increased over the predetermined pressure, corresponding one of upper and lower portions of plate spring 104 bends into either recess 68′a or 68′b, thereby release the pressurized oil to the oil sump. Thus, plate spring 104 functions as a set of upper and lower twin relief valves which allow the oil pressured over the limit in respective oil through-holes 98U and 98L to escape. Plate spring 104 may be replaced with another having a differential spring force to alter the hydraulic pressure required for relief However, in this embodiment, the relief valves constituted by plate spring 104 are determined with respect to their relief pressure so that they are still closed while the hydraulic pressure in the HST being kept within its usual range, but one of them is opened by excessive residual pressure instantly generated in either oil passage 5′a or 5′b when the HST is returned to the neutral position rapidly.

Now, control arm 39′ which has been tilted is operated for deceleration and comes to the neutral position or its vicinity so as to bring oil supply/discharge ports 82U and 82L into communication with respective oil through-passages 98U and 98L. Residual pressured oil in either oil passage 5′a or 5′b, if remaining therein, flows into either oil through-passage 98U or 98L through either oil supply/discharge port 82U and 82L. Then, if the inner-pressure of either oil through-passage 98U or 98L into which the residual oil flows is increased over the predetermined pressure, corresponding one of the upper and lower portions of plate spring 104 is elastically bent so as to drain the excessive pressured oil in control arm 39′ to the oil sump. Due to such releasing of the excessive residual pressure, the jerkiness generated on stopping is moderated. If the hydraulic pressure in either oil through-passage 98U or 98L increased by the residual pressure in either oil passage 5′a or 5′b does not reach the predetermined pressure, plate spring 104 is not bent and still shuts oil through-passages 98U and 98L from the oil sump, whereby the residual pressured oil flows into the other of oil through-passages 98U and 98L via pin hole 100 a and slits 100 b of orifice member 100 so as to be bypassed between oil passages 5′a and 5′b from the higher-pressured to the depressed. Due to such bypassing of the residual pressured oil between oil through-passages 98U and 98L, hydraulic motor 47 becomes stationary so as to bring the HST into the neutral condition, whereby the vehicle stops certainly.

Either oil supply/discharge port 82U or 82L, either oil through-passage 98U or 98L and connection hole 101 is brought into communication with the oil sump only when the pressure in either oil through-passage 98U or 98L is increased over the predetermined pressure so as to push plate spring 104. If one of oil through-passages 98U and 98L is depressed, it is still shut from the oil sump by plate spring 104. Therefore, oil having air bubbles is not absorbed into the depressed one of oil passages 5′a and 5′b in the housing, but the oil circulated in center section 5′ and control arm 39′ is absorbed. Thus, penetration of air bubbles into oil passages 5′a and 5′b in center section 5′ is avoided.

By increasing the diameter of pin hole 100 a, the faster the residual pressured oil flows from the high-pressured to the depressed. Therefore, the shock on stopping of the vehicle is more effectively reduced. Also, the dead zone of the HST in the vicinity of its neutral position can be extended so as to establish the neutral condition of the HST more certainly.

However, if the diameter of pin hole 100 a is excessively large, the oil is bypassed via orifice member 100 between oil passages 5′a and 5′b too easily. When the vehicle with the HST being in neutral is left on a slope, the hydraulic pressure in one of oil passages 5′a and 5′b is easily increased while that in the other being decreased so as to generate the difference of hydraulic pressure between oil passages 5′a and 5′b, thereby rotating motor shaft 4. As a result, the vehicle descends the slope. Thus, the diameter of pin hole 100 a is determined in consideration of such mutual contradictory conditions.

FIGS. 72(a) to 73(g) illustrate appearances of control arm 39′ during its rotation. Referring to FIG. 72(a), control arm 39′ is located at the neutral position, wherein upper oil through-passage 98U communicates with upper oil supply/discharge port 82U, and lower oil through-passage 98L communicates with lower oil supply/discharge port 82L.

Now, control arm 39′ is rotated for advancing of the vehicle from the neutral position. When control arm 39′ comes to the position as shown in FIG. 72(b), wherein a line passing both axes of oil through-passages 98U and 98L is rotated from above-mentioned neutral line NL at an angle +α, lower oil through-passage 98L is separated away from lower supply/discharge port 82L.

Control arm 39′ is further rotated, and then, upper oil through-passage 98U is also separated away from upper supply/discharge port 82U as shown in FIG. 72(c), wherein the line passing both axes of oil through-passages 98U and 98L is rotated from neutral line NL at an angle +β, which is larger than angle +α.

The diameters of openings of oil through-passages 98U and 98L at the front surface of control plate 39′c are equal to each other, while those of oil supply/discharge ports 82U and 82L are equal to each other. However, upper oil through-passage 98U is located nearer to the fulcrum of control arm 39′ than lower oil through-passage 98L. As a result, the rotational range (from 0 up to β) of control arm 39′ for communicating upper oil through-passage 98U with upper oil supply/discharge port 82U is wider than that (from 0 up to α) for communicating lower oil through-passage 98L with lower oil supply/discharge port 82L.

Next, control arm 39′ is rotated for reversing of the vehicle from the neutral position shown in FIG. 72(a). Similarly with the operation for advancing the vehicle, firstly, lower oil through-passage 98L is separated away from lower oil supply/discharge port 82L when control arm 39′ is rotated at an angle −α from the neutral position as shown in FIG. 73(e), and afterwards, upper oil through-passage 98U is separated away from upper oil supply/discharge port 82U when control arm 39′ is rotated at an angle −β from the neutral position as shown in FIG. 73(f).

The above-mentioned bypassing with the medium of orifice member 100 is performed only when both upper and lower oil through-passages 98U and 98L communicate with respective upper and lower oil supply/discharge ports 82U and 82L. In other words, the rotational angle range of control arm 39′ for bypassing between oil passages 5′a and 5′b is set between −α and +α through 0.

On the other hand, the above-mentioned relief of oil with the medium of plate spring 104 is performed when either higher-pressured oil supply/discharge port 82U or 82L communicates with corresponding one of oil through-passages 98U and 98L. In this embodiment, when control arm 39′ is returned from its range for advancing the vehicle to the neutral position, upper oil passage 5′a is higher-pressured with back-pressure caused by the pumping actuation of hydraulic motor 46 which is rotated according to the inertia rotation of axles 7. On the contrary, when control arm 39′ is returned from its range for reversing the vehicle to the neutral position, lower oil passage 5′a is higher-pressured because of the similar condition of hydraulic motor 46. As a result, the rotational angle range of control arm 39′ for releasing oil from oil passages 5′a and 5′b is set between −α and +β through 0.

In such a manner, control arm 39 is now rotated for advancing the vehicle from the neutral position. In the vicinity of the neutral position, upper and lower oil through-passages 98U and 98L communicate with respective upper and lower oil supply/discharge ports 82U and 82L so that oil is bypassed between oil passages 5′a and 5′b via orifice member 100. Consequently, the rising of hydraulic pressure in oil passage 5′b to be higher-pressured is delayed relative to the rotation of control arm 39′, whereby hydraulic motor 46 starts driving smoothly so as to moderate the shock on starting of the vehicle.

Control arm 39′ is further rotated up to the position shown in FIG. 72(b) so as to increase the hydraulic pressure in oil passage 5′b and oil through-passage 98L. However, the rising of hydraulic pressure therein is limited so that plate spring 104 is not pushed to bend and oil through-passages 98U and 98L are still shut from the oil sump in the housing.

When rotated control arm 39 reaches the position shown in FIG. 72(b), oil supply/discharge port 82L is isolated from oil through-passage 98L and closed by the front surface of contact plate 39′c. Thus, higher-pressured oil passage 5′b is no longer subjected to the effect of oil-releasing or oil-bypassing. This means that the HST can be actuated effectively to the maximum capacity of its hydraulic oil circulation between oil passages 5′a and 5′b when control arm 39′ is rotated away from its neutral setting range.

Control arm 39′ is further rotated from the position shown in FIG. 72(b) so that lower piston 64L is thrust outward by higher-pressured oil in lower oil passage 5′b. Contact plate 39′c, while shutting oil supply/discharge port 82L, is pressed against friction plate 68′ so as to generate friction force which resists against the rotation of control arm 39′. Contact plate 39′ must be rotated by an operation force exceeding resistance force involving this friction force.

Now, control arm 39′ which has been rotated to the side for advancing the vehicle is released from the operation force and returned by biasing force of neutral spring 31. During this returning of control arm 39′, hydraulic motor 46 functions as a pump by following inertially rotated axles 7 so that upper oil passage 5′a is higher-pressured and lower oil passage 5′b is depressed. Consequently, upper piston 64U is thrust outward by the higher-pressured oil in upper oil passage 5′a so that contact plate 39′c, while shutting oil supply/discharge port 82U, is pressed against friction plate 68′ so as to generate friction force which resists against the neutral-returning of control arm 39′. Therefore, control arm 39′ is gradually returned to the neutral position while being braked so as to avoid sudden dynamic braking.

When returning control arm 39′ reaches the position shown in FIG. 72(c), upper oil supply/discharge port 82U is brought into communication with upper oil through-passage 98U. If control arm 39′ which has been rotated to a high-speed position in advancing of the vehicle is rapidly returned to the neutral position, the higher-pressured oil in upper oil passage 5′a is reduced but the residual pressure remains therein. In the position shown in FIG. 72(c), when the residual pressure exceeds the predetermined pressure, plate spring 104 is bent as at phantom lines in FIG. 68 so as to open upper oil through-passage 98U to the oil sump, whereby the pressured oil in oil through-passage 98U from oil passage 5′a is released to the oil sump.

Thus, dynamic braking is also avoided in this case. Control arm 39′ is further returned from the position shown in FIG. 72(c) so as to reach the position shown in FIG. 72(b) so that lower oil supply/discharge port 82L also communicates with lower oil through-passage 98L. The higher-pressured oil in oil passage 5′a is bypassed through orifice member 100 into depressed oil passage 5′b, whereby both oil passages 5′a and 5′b are equalized in hydraulic pressure. Consequently, the vehicle stops moderately, when control arm 39′ is rotated from the neutral position for reversing the vehicle and then returned to the neutral position, vice versa.

Description will now be given on a first modification of the sixth embodiment in accordance with FIGS. 75 to 77. This is structured such that the orifice is automatically opened and closed for solving the problem that the vehicle with the HST in neutral left on a slope descends the slope because of the inability to keep the hydraulic pressure in the hydraulic oil circuit of the HST.

As shown in FIG. 75, a connection hole 101′ replacing connection hole 101 is drilled upwardly into contact plate 39′c through both oil through-passages 98U and 98L. Instead of orifice member 100, a valve unit 106 is inserted into connection hole 101′. Valve unit 106 is constituted by a cylindrical valve casing 108, a piston 107, a pair of springs 109, and a threaded cap 103. One closed end of valve casing 108 abuts against the most recessed or top end of connection hole 101′. Cap 103 covers the other open end of valve casing 108 and plugs the bottom opening of connection hole 101′. Piston 107 is slidably but hydraulically-tightly disposed in valve casing 108 so as to divide the inner chamber of valve casing 108 into a pair of upper and lower oil chambers CU and CL. In valve casing 108 are interposed springs 109 between the ceiling of valve casing 108 and the top surface of piston 107, and between the bottom surface of piston 107 and the top surface of cap 103, respectively. Both springs 109 apply biasing forces in opposite directions onto piston 107, so as to balance piston 107 at an optimal position.

Piston 107 is bored therein with a pair of axial holes 107 a which are separated from each other and open to respective oil chambers CU and CL. Piston 107 is also bored therein with a pair of radial holes 107 b serving as orifices, each of which crosses each of axial holes 107 a and has opposite open ends at the peripheral surface of piston 107. Valve casing 108 is formed at its inner periphery with an annular groove 108 a.

When piston 107 is balanced between springs 109 as mentioned above, both upper and lower radial holes 107 b communicate with annular groove 108 a. If piston 107 slides upward or downward to some degree from the balancing position, one of radial holes 107 b is isolated from annular groove 108 a so that both radial holes 107 b are fluidly disconnected from each other, thereby separating oil chambers CU and CL from each other.

Valve casing 108 is bored therein with a pair of upper and lower radial holes 108 b, each of which penetrates each of oil chambers CU and CL and is open to each of oil through-passages 98U and 98L. Therefore, oil pressured higher in either oil through-passage 98U or 98L is introduced into corresponding one of oil chambers CU and CL so that piston 107 slides in the direction and degree determined by the difference of hydraulic pressure between upper and lower oil chambers CU and CL.

In this structure, when control arm 39′ is located in the neutral position and the vehicle is left on a slope, axles 7 are intended to rotate for descending the slope by the self-weight of the vehicle, so that a torque is applied onto motor shaft 4 interlocking with axles 7, thereby making hydraulic motor 46 function as a pump so as to generate a difference of hydraulic pressure between oil passages 5′a and 5′b. Since control arm 39′ is at the neutral position, both oil passages 5′a and 5′b communicate with respective oil through-passages 98U and 98L through respective oil supply/discharge ports 82U and 82L so that between oil chambers CU and CL is generated a difference of hydraulic pressure, which is the same with that between oil passages 5′a and 5′b.

Accordingly, piston 107 slides so as to disconnect both radial holes 107 b from each other, whereby oil is not bypassed between oil passages 5′a and 5′b. Thus, the difference of hydraulic pressure between oil passages 5′a and 5′b is kept and functions as a braking force which stops the rotation of motor shaft 4. As a result, the vehicle is securely prevented from unintentionally descending the slope.

Even when the vehicle is stationary on flat ground while control arm 39′ is located at the neutral position, the difference of hydraulic pressure between oil passages 5′a and 5′b may be generated by a positional difference of swash plate 11 from its proper neutral position because of an error of linkage or the like. However, the difference of hydraulic pressure is too slight to move piston 107 for completely cutting the fluidal communication between both radial holes 1 07 b. Therefore, oil is bypassed from higher-pressured one of oil passages 5′a and 5′b to the other depressed one. Consequently, if control arm 39′ is located at the neutral position, the vehicle is securely kept stationary without slight movement. Since the HST is provided with such an extended dead zone in the vicinity of its neutral position, the vehicle can be easily operated and its neutral setting may be easily adjusted.

In this state, into depressed oil passage 5′a or 5′b is absorbed not oil from the oil sump but hydraulic oil from higher-pressured oil passage 5′b or 5′a through oil through-passages 98U and 98L and valve unit 106. Accordingly, there is no room for air bubbles to penetrate into oil passages 5′a and 5′b as a closed fluid circuit of the HST. This is similar to the above-mentioned embodiment.

If control arm 39′ is rapidly returned to the neutral position from high-speed traveling condition, either oil passage 5′a or 5′b is hydraulically pressured excessively. However, plate spring 104 is elastically bent so as to release the excessively pressured oil from either oil passage 5′a or 5′b to the oil sump through either oil through-passage 98U or 98L, thereby avoiding the generation of sudden dynamic braking. Such prevention of jerkiness is performed similarly as with the above-mentioned embodiment.

Next, description will be given on a second modification of the sixth embodiment in accordance with FIGS. 78 to 81. This is structured to enable not only oil bypassed from higher-pressured oil passage 5′a or 5′b through an orifice but also oil, which is excessively high-pressured suddenly in either oil passages 5′a or 5′b and drained through a relief valve, to be absorbed into depressed oil passage 5′a or 5′b when control arm 39′ is located at the neutral position.

As shown in FIGS. 78 and 79, the back surface of contact plate 39′c of control arm 39′ abutting against friction plate 68 is formed with a rectangular groove 110 which is elongated along the radial direction of control shaft 35. Groove 110 is revolved centering on control shaft 35 according to the rotation of control arm 39′. However, groove 110 is narrowed to be closed by friction plate 68 even if control arm 39′ is rotated to the limit. Alternatively, positionally-fixed friction plate 68 may be sufficiently wide so as to close groove 110 constantly. The bottom surface of groove 110 is flat and upper and lower oil through-passages 98U and 98L are open at the bottom surface of groove 110.

To tell the difference from the above-mentioned embodiment, plate spring 104 is not provided on friction plate 68 but fastened with a bolt on the bottom surface of groove 110 of contact plate 39′c so as to cover the open ends of upper and lower through-passages 98U and 98L thereon. Plate spring 104 is formed with a plurality of small openings 104 a in correspondence to each of openings of upper and lower oil through-passages 98U and 98L. A pair of upper and lower half-spherical caps 112 are fixed on the outer surface of plate spring 104 by welding so as to cover openings 104 a. Each of caps 112 is formed at its top with an opening 112 a. A steel ball 111 as a valve is disposed in each of caps 112. Opening 104 a of plate spring 104 and openings 112 a of cap 112 are diametrically smaller than steel ball 111 so as to prevent steel ball 111 from escaping through opening 104 a or 112 a.

A pair of check valves 113U and 113L consisting of plate spring 104, caps 112 and steel balls 111 in the above-mentioned arrangement check the flow of oil from oil through-passages 98U and 98L to groove 110, and allows flow from groove 110 to oil through-passages 98U and 98L.

A round rod 100′ replacing orifice member 100 is inserted into connection hole 101 interposed between both oil through-holes 98U and 98L.

Round rod 100′ is slightly diametrically smaller than connection hole 101 so that the gap between round rod 100′ and connection hole 101 serves as an orifice 114. Round rod 100′ has the same effect with orifice member 100 such as to reduce the flowing amount of oil bypassed through connection hole 101. However, round rod 100′ is simpler than orifice member 100, thereby reducing costs.

Referring to FIG. 78 illustrating the above-mentioned structure in the instance when control arm 39′ is turned toward the neutral position from the condition for advancing the vehicle, hydraulic pump 46 is driven by inertial force to function as a pump. Therefore, upper oil passage 5′a is hydraulically higher-pressured and lower oil passage 5′b is depressed. Upper check valve 113U on the upper portion of plate spring 104 is closed so as to prevent oil in higher-pressured oil passage 5′a from escaping through oil through-passage 98U. Lower check valve 113L on the lower portion of plate spring 104 is open. However, groove 110 is covered with friction plate 68 so as to be shut from the oil sump, whereby pressured oil is not endlessly absorbed into depressed lower oil passage 5′b from lower check valve 113L.

If, when the vehicle is advancing at high speed, control arm 39′ is rapidly returned to the neutral position, excessive residual pressure remains in upper oil passage 5′a. When control arm 39′ reaches the neutral position, upper and lower oil passages 5′a and 5′b communicates with respective upper and lower oil supply/discharge ports 82U and 82L. Upper oil passage 5′a is still higher-pressured by the residual pressure. Therefore, upper check valve 113U is closed, however, plate spring 104 is bent as in phantom lines in FIG. 78 so as to release the oil in oil through-passage 98U to groove 110 when the residual pressure exceeds the spring force of plate spring 104. Pressured oil escaping into groove 110 is introduced into oil through-passage 98L through check valve 113L and absorbed into depressed oil passage 5′b. Consequently, the excessive residual pressure is released so that the vehicle stops moderately.

Depressed oil through-passage 98L absorbs oil bypassed from higher-pressured oil passage 5′a through orifice 114, or additionally, through the open end of upper oil through-passage 98U which is opened with bent plate spring 104 functioning as a relief valve 104, groove 110 and lower check valve 113L. Depressed oil through-passage 98L does not absorb oil having air bubbles from the oil sump in the housing because groove 110, oil through-passages 98U and 98L and orifice 114 are closed from the oil sump.

In this structure, excessively pressured oil flowing into groove 110 through opened relief valve 104 from one of oil through-passages 98U and 98L can be absorbed swiftly into the other of oil through-passages 98U and 98L, thereby preventing hydraulic oil circulated between oil passages 5′a and 5′b from being reduced.

More specifically, in the foregoing sixth embodiment as shown in FIG. 68 and its first modification as shown in FIG. 75, relief valve 104 provided on friction plate 68′ is bent to open, so as to drain the excessively pressured oil to the oil sump once. Then, oil passages 5′a and 5′b as a closed oil circuit of the HST is supplied with oil from the oil sump, which is absorbed by depressed oil passage 5′a or 5′b through filter 56, supply oil passage 5′c and either check valve 54 or 55. Thus, decrease of hydraulic oil can be prevented. However, if the residual pressure is great so that a large quantity of hydraulic oil is released at a time, it takes a long time to save the loss of hydraulic oil completely. Therefore, cavitation is generated by the temporary reduction of hydraulic oil. The second modification of the sixth embodiment solving this problem is advantageous in that the certainty of hydraulic oil between oil passages 5′a and 5′b can be maintained. 

What is claimed is:
 1. A hydrostatic transmission immersed in an oil sump formed in a housing, comprising: a hydraulic pump; a hydraulic motor; a center section on which said hydraulic pump and said hydraulic motor are mounted, wherein said center section is formed therein with a pair of oil passages through which said hydraulic pump and said hydraulic motor are fluidly connected with each other, and said center section is provided on an outer surface thereof with a pair of openings of said respective oil passages, a pump capacity changing member for varying the direction and amount of hydraulic fluid discharged from said hydraulic pump to said hydraulic motor through said pair of oil passages in said center section; a control shaft rotatably supported by said housing; a control arm disposed in said housing, said control arm being rotatably centered on said control shaft and engaged with said pump capacity changing member, wherein said pump capacity changing member is moved to various actuation positions including a neutral position in association with rotation of said control arm rotated to various actuation positions including a neutral position; a contact member formed integrally with said control arm, said contact member having a surface abutting against said pair of openings of said center section; a pair of oil passages provided in said contact member, wherein said pair of oil passages in said contact member are shut from said oil sump and open on said surface of said contact member, and wherein said pair of oil passages in said contact member communicate with said respective openings of said center section when said control arm is located in a certain range including said neutral position; and an orifice provided in said contact member so as to connect said pair of oil passages in said contact member with each other.
 2. The hydrostatic transmission as set forth in claim 1, wherein each of said oil passages in said contact member has substantially the same diameter as that of a corresponding one of said openings of said oil passages in said center section.
 3. The hydrostatic transmission as set forth in claim 1, wherein said pair of openings of said oil passages in said center section are juxtaposed in a radial direction centering on an axis of said control shaft.
 4. A hydrostatic transmission immersed in an oil sump formed in a housing, comprising: a hydraulic pump; a hydraulic motor; a center section on which said hydraulic pump and said hydraulic motor are mounted, wherein said center section is formed therein with a pair of oil passages through which said hydraulic pump and said hydraulic motor are fluidly connected with each other, and said center section is provided on an outer surface thereof with a pair of openings of said respective oil passages; a pump capacity changing member for varying the direction and amount of hydraulic fluid discharged from said hydraulic pump to said hydraulic motor through said pair of oil passages in said center section; a control shaft rotatably supported by said housing; a control arm disposed in said housing, said control arm being rotatably centered on said control shaft and engaged with said pump capacity changing member, wherein said pump capacity changing member is moved to various actuation positions including a neutral position in association with rotation of said control aim rotated to various actuation positions including a neutral position; a contact member formed integrally with said control arm, said contact member having a pair of first and second surfaces opposite to each other, said first surface of said contact member abutting against said pair of openings of said center section; a pair of oil passages provided through said contact member between said first and second surfaces, wherein said pair of oil passages are open on both said first and second surface of said contact member, and wherein said pair of oil passages in said contact member communicate with said respective openings of said center section when said control arm is located in a certain range including said neutral position; an orifice provided in said contact member so as to connect said pair of oil passages in said contact member with each other; a block fixed to said housing, said block having a frictional surface abutting against said second surface of said contact member; and a pair of relief valves provided on said block corresponding to said respective oil passages in said contact member, wherein each of said relief valves is opened so as to release oil from corresponding one of said oil passages in said contact member to said oil sump when corresponding one of said oil passages in said contact member is hydraulically pressured higher than a certain pressure while said oil passage in said contact member communicates with corresponding one of said oil passages in said center section.
 5. The hydrostatic transmission as set forth in claim 4, wherein said pair of relief valves are hydraulically controlled.
 6. The hydrostatic transmission as set forth in claim 5, wherein said pair of relief valves are constituted by a plate spring provided on said block so as to abut against said openings of oil passages on said second surface of said contact member, and wherein said plate spring is bent so as to bring one of said oil passages in said contact member hydraulically pressured higher than said certain pressure into communication with said oil sump.
 7. The hydrostatic transmission as set forth in claim 6, wherein a surface of said plate spring forms a common flat surface together with said frictional surface of said block.
 8. A hydrostatic transmission immersed in an oil sump formed in a housing, comprising: a hydraulic pump; a hydraulic motor; a center section on which said hydraulic pump and said hydraulic motor are mounted, wherein said center section is formed therein with a pair of oil passages through which said hydraulic pump and said hydraulic motor are fluidly connected with each other, and said center section is provided on an outer surface thereof with a pair of openings of said respective oil passages; a pump capacity changing member for varying the direction and amount of hydraulic fluid discharged from said hydraulic pump to said hydraulic motor through said pair of oil passages in said center section; a control shaft rotatably supported by said housing; a control arm disposed in said housing, said control arm being rotatably centered on said control shaft and engaged with said pump capacity changing member, wherein said pump capacity changing member is moved to various actuation positions including a neutral position in association with rotation of said control arm rotated to various actuation positions including a neutral position; a contact member formed integrally with said control arm, said contact member having a pair of first and second surfaces opposite to each other, said first surface of said contact member abutting against said pair of openings of said center section; a pair of oil passages provided in said contact member between said first and second surfaces, wherein said pair of oil passages are open on said first surface of said contact member, and wherein said pair of oil passages in said contact member communicate with said respective openings of said center section when said control arm is located in a certain range including said neutral position; an orifice provided in said contact member so as to connect said pair of oil passages in said contact member with each other; a block fixed to said housing, said block having a frictional surface abutting against said second surface of said contact member; a groove formed on said second surface of said contact member, wherein said groove is shut by said block from said oil sump; a pair of check valves provided in said contact member, each of said check valves being interposed between each of said oil passages in said contact member and said groove, wherein each of said check valves allows oil to flow in one way from said groove to each of said oil passages in said contact member; and a pair of relief valves provided in said contact member, each of said relief valves having each of said check valves, wherein each of said relief valves is opened so as to release oil from corresponding one of said oil passages in said contact member to said groove when corresponding one of said oil passages in said contact member is hydraulically pressured higher than a certain pressure while said oil passage in said contact member communicates with corresponding one of said oil passages in said center section.
 9. The hydrostatic transmission as set forth in claim 8, wherein said pair of relief valves are hydraulically controlled.
 10. The hydrostatic transmission as set forth in claim 8, wherein said pair of relief valves are constituted by a plate spring, and wherein said plate spring initially shuts each of said oil passages in said contact member from said groove and is bent so as to bring one of said oil passages in said contact member hydraulically pressured higher than said certain pressure into communication with said groove. 